erts: Remove +R from documentation

The +R flag has not been used since 2012, so we remove it
from the docs but keep it in the code in case we ever need
to use it again in the future.

Author: garazdawi

erts/doc/references/erl_cmd.md

@@ -65,14 +65,14 @@ arguments_:
 _Examples:_
 
 ```erlang
-% erl +W w -sname arnie +R 9 -s my_init -extra +bertie
+% erl +W w -sname arnie +S 2 -s my_init -extra +bertie
 (arnie@host)1> init:get_argument(sname).
 {ok,[["arnie"]]}
 (arnie@host)2> init:get_plain_arguments().
 ["+bertie"]
 ```
 
-Here `+W w` and `+R 9` are emulator flags. `-s my_init` is an init flag,
+Here `+W w` and `+S 2` are emulator flags. `-s my_init` is an init flag,
 interpreted by `init`. `-sname arnie` is a user flag, stored by `init`. It is
 read by Kernel and causes the Erlang runtime system to become distributed.
 Finally, everything after `-extra` (that is, `+bertie`) is considered as plain
@@ -856,21 +856,6 @@ behavior of earlier flags.
   On Windows the default value is set to `8196` because the normal OS
   limitations are set higher than most machines can handle.
 
-- **`+R ReleaseNumber`{: #compat_rel }** - Sets the compatibility mode.
-
-  The distribution mechanism is not backward compatible by default. This flag
-  sets the emulator in compatibility mode with an earlier Erlang/OTP release
-  `ReleaseNumber`. The release number must be in the range
-  `<current release>-2` through `<current release>`. This limits the emulator,
-  making it possible for it to communicate with Erlang nodes (as well as C
-  and Java nodes) running that earlier release.
-
-  > #### Note {: .info }
-  >
-  > Ensure that all nodes (Erlang-, C-, and Java nodes) of a distributed Erlang
-  > system is of the same Erlang/OTP release, or from two different Erlang/OTP
-  > releases X and Y, where _all_ Y nodes have compatibility mode X.
-
 - **`+r`** - Forces ETS memory blocks to be moved on reallocation.
 
 - **`+rg ReaderGroupsLimit`{: #+rg }** - Limits the number of reader groups used

erts/etc/common/erlexec.c

@@ -1279,7 +1279,6 @@ usage_aux(void)
 	  "[+C MODE] [+dcg DECENTRALIZED_COUNTER_GROUPS_LIMIT] [+h HEAP_SIZE_OPTION] "
           "[+J[Pperf|Msingle] JIT_OPTION] "
 	  "[+M<SUBSWITCH> <ARGUMENT>] [+P MAX_PROCS] [+Q MAX_PORTS] "
-	  "[+R COMPAT_REL] "
 	  "[+r] [+rg READER_GROUPS_LIMIT] [+s<SUBSWITCH> SCHEDULER_OPTION] "
 	  "[+S NO_SCHEDULERS:NO_SCHEDULERS_ONLINE] "
 	  "[+SP PERCENTAGE_SCHEDULERS:PERCENTAGE_SCHEDULERS_ONLINE] "