DOCS: add for CONCURRENCY_VERIFIED (#3510)

Co-authored-by: Tom Limoncelli <[email protected]>

Author: gotjoshua

documentation/language-reference/CONCURRENCY_VERIFIED.md

@@ -0,0 +1,16 @@
+---
+name: CONCURRENCY_VERIFIED
+---
+
+✅  - A checkmark means "this has been tested, and will run concurrently".  
+❔  - The questionmark means "it hasn't been tested, safety unknown"  
+❌  - The red "X" means "this has been tested, and it will _not_ work concurrently".  
+
+
+### Here's how to perform a test:
+
+1. Build dnscontrol with the -race flag: go build -race (this makes a special binary)
+2. Run this special binary: `dnscontrol preview` with a configuration that lists at least 4 domains using Porkbun. More domains is better.
+3. The special binary runs much slower (1/10th the speed) because it is doing a lot of checking. 
+If it reports problems, the error message will indicate where the problem is. 
+It might not be 100% accurate, but it will be in the right area.

documentation/provider/index.md

@@ -12,7 +12,7 @@ a provider that supports it, we'd love your contribution to ensure it works corr
 If a feature is definitively not supported for whatever reason, we would also like a PR to clarify why it is not supported, and fill in this entire matrix.
 
 <!-- provider-matrix-start -->
-| Provider name | Official Support | DNS Provider | Registrar | Concurrency Verified | [`ALIAS`](../language-reference/domain-modifiers/ALIAS.md) | [`CAA`](../language-reference/domain-modifiers/CAA.md) | [`AUTODNSSEC`](../language-reference/domain-modifiers/AUTODNSSEC_ON.md) | [`HTTPS`](../language-reference/domain-modifiers/HTTPS.md) | [`LOC`](../language-reference/domain-modifiers/LOC.md) | [`NAPTR`](../language-reference/domain-modifiers/NAPTR.md) | [`PTR`](../language-reference/domain-modifiers/PTR.md) | [`SOA`](../language-reference/domain-modifiers/SOA.md) | [`SRV`](../language-reference/domain-modifiers/SRV.md) | [`SSHFP`](../language-reference/domain-modifiers/SSHFP.md) | [`SVCB`](../language-reference/domain-modifiers/SVCB.md) | [`TLSA`](../language-reference/domain-modifiers/TLSA.md) | [`DS`](../language-reference/domain-modifiers/DS.md) | [`DHCID`](../language-reference/domain-modifiers/DHCID.md) | [`DNAME`](../language-reference/domain-modifiers/DNAME.md) | [`DNSKEY`](../language-reference/domain-modifiers/DNSKEY.md) | dual host | create-domains | get-zones |
+| Provider name | Official Support | DNS Provider | Registrar | [Concurrency Verified](../language-reference/domain-modifiers/CONCURRENCY_VERIFIED.md) | [`ALIAS`](../language-reference/domain-modifiers/ALIAS.md) | [`CAA`](../language-reference/domain-modifiers/CAA.md) | [`AUTODNSSEC`](../language-reference/domain-modifiers/AUTODNSSEC_ON.md) | [`HTTPS`](../language-reference/domain-modifiers/HTTPS.md) | [`LOC`](../language-reference/domain-modifiers/LOC.md) | [`NAPTR`](../language-reference/domain-modifiers/NAPTR.md) | [`PTR`](../language-reference/domain-modifiers/PTR.md) | [`SOA`](../language-reference/domain-modifiers/SOA.md) | [`SRV`](../language-reference/domain-modifiers/SRV.md) | [`SSHFP`](../language-reference/domain-modifiers/SSHFP.md) | [`SVCB`](../language-reference/domain-modifiers/SVCB.md) | [`TLSA`](../language-reference/domain-modifiers/TLSA.md) | [`DS`](../language-reference/domain-modifiers/DS.md) | [`DHCID`](../language-reference/domain-modifiers/DHCID.md) | [`DNAME`](../language-reference/domain-modifiers/DNAME.md) | [`DNSKEY`](../language-reference/domain-modifiers/DNSKEY.md) | dual host | create-domains | get-zones |
 | ------------- | ---------------- | ------------ | --------- | -------------------- | ---------------------------------------------------------- | ------------------------------------------------------ | ----------------------------------------------------------------------- | ---------------------------------------------------------- | ------------------------------------------------------ | ---------------------------------------------------------- | ------------------------------------------------------ | ------------------------------------------------------ | ------------------------------------------------------ | ---------------------------------------------------------- | -------------------------------------------------------- | -------------------------------------------------------- | ---------------------------------------------------- | ---------------------------------------------------------- | ---------------------------------------------------------- | ------------------------------------------------------------ | --------- | -------------- | --------- |
 | [`AKAMAIEDGEDNS`](akamaiedgedns.md) | ❌ | ✅ | ❌ | ❔ | ❌ | ✅ | ✅ | ❔ | ✅ | ✅ | ✅ | ❌ | ✅ | ✅ | ❔ | ✅ | ❌ | ❔ | ❔ | ❔ | ✅ | ✅ | ✅ |
 | [`AUTODNS`](autodns.md) | ❌ | ✅ | ❌ | ✅ | ✅ | ✅ | ❔ | ❔ | ❔ | ❔ | ✅ | ❔ | ✅ | ❌ | ❔ | ❌ | ❌ | ❔ | ❔ | ❔ | ❌ | ❌ | ✅ |