fix: rewrite SI table filtering guidance and add rewrite requirement

The existing SI paragraph (added in PR #11) had several style issues:
vague framing, a confusing "PMT-centric" label, and no guidance on
rewriting SDT/EIT when derived from an MPTS source.

* Replace the paragraph with a clear explanation: the PMT references
  only program elementary-stream PIDs and never references the fixed
  service information tables (NIT, SDT/BAT, EIT, TDT/TOT); the filter
  therefore drops them silently.
* Expand all acronyms on first use.
* Split the two SHOULD obligations (retain, declare) into separate
  sentences.
* Add rewrite guidance: because SDT and EIT from an MPTS describe all
  programs, publishers SHOULD filter or rewrite them to include only
  entries for the carried program. NIT and TDT/TOT are broadcast-wide
  and do not require per-program rewriting.

Addresses issue #13.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: gwendalsimon

draft-gregoire-moq-msfts.md

@@ -268,15 +268,21 @@ scrambled transport streams MUST also retain the conditional access packets
 required for descrambling; conditional access integration is application-specific
 and outside the scope of this document.
 
-The retain list above is PMT-centric.  DVB and ATSC broadcast standards carry
-service information on fixed well-known PIDs that the PMT does not reference:
-SDT/BAT (0x0011), EIT (0x0012), TDT/TOT (0x0014), and NIT (0x0010), or their
-ATSC PSIP equivalents.  The filter silently drops these tables, leaving the
-resulting track without service identity, EPG, or broadcast time.  Publishers
-targeting broadcast or IRD deployments SHOULD retain the SI tables required by
-the target standard.  When SI tables are retained, publishers SHOULD declare the
-additional PIDs using `m2tsSiPids` ({{m2ts-si-pids}}) so that subscribers can
-verify which tables are present.
+The Program Map Table references only the PIDs of the selected program's
+elementary streams and PCR; it does not reference the service information
+tables defined by DVB and ATSC: Network Information Table (NIT, 0x0010),
+Service Description Table and Bouquet Association Table (SDT/BAT, 0x0011),
+Event Information Table (EIT, 0x0012), and Time and Date Table with Time
+Offset Table (TDT/TOT, 0x0014), or their ATSC PSIP equivalents.  The filter
+therefore drops these tables, leaving the resulting track without service
+identity, EPG, or broadcast time.  Publishers producing tracks for broadcast
+or IRD reception SHOULD retain the SI tables required by the target standard.
+When SI tables are retained, publishers SHOULD declare the additional PIDs
+using `m2tsSiPids` ({{m2ts-si-pids}}) so that subscribers can verify which
+tables are present.  Because SDT and EIT carried from an MPTS describe all
+programs in the multiplex, publishers SHOULD filter or rewrite these tables
+to include only the service and schedule entries for the carried program;
+NIT and TDT/TOT are broadcast-wide and do not require per-program rewriting.
 
 The `m2tsProgramNumber` field ({{m2ts-program-number}}) SHOULD be present on
 per-program tracks to identify the program carried.  When multiple per-program