Update simple_switch docs for better support of if in action bodies (#1278)

Signed-off-by: Andy Fingerhut <[email protected]>

Author: jafingerhut

docs/simple_switch.md

@@ -800,28 +800,18 @@ by the design of the hardware.
 
 ### Restrictions on code within actions
 
-These restrictions are actually restrictions of the `p4c` compiler, not of
-`simple_switch`.  Anyone interested in enhancing `p4c` to remove these
-restrictions should see the issues below.
+After 2024-Nov-08 when this change was made to the `p4c` BMv2 back
+end:
 
-The P4_16 language specification v1.1.0 permits `if` statements within action
-declarations.  `p4c`, when compiling for the target BMv2 simple_switch, supports
-some kinds of `if` statements, in particular ones that can be transformed into
-assignments using the ternary `condition ? true_expr : false_expr` operator.
-This is supported:
++ https://github.com/p4lang/p4c/pull/4999
 
-```
-    action foo() {
-        meta.b = meta.b + 5;
-        if (hdr.ethernet.etherType == 7) {
-            hdr.ethernet.dstAddr = 1;
-        } else {
-            hdr.ethernet.dstAddr = 2;
-        }
-    }
-```
+arbitrary `if` statements, even `if` statements nested within other
+`if` statements, should be supported correctly.
 
-but this is not, as of 2022-Jan-18:
+Before that change was made to `p4c`, there were significant
+restrictions on the kinds of `if` statements that were supported
+within the bodies of a P4 `action`, when compiling for BMv2.  For
+example, the following `action` definition was not supported:
 
 ```
     action foo() {
@@ -834,24 +824,6 @@ but this is not, as of 2022-Jan-18:
     }
 ```
 
-Given the following text from the P4_16 language specification, it is likely
-that there are other P4 implementations that have limited or no support for `if`
-statements within actions:
-
-    No `switch` statements are allowed within an action --- the grammar permits
-    them, but a semantic check should reject them.  Some targets may impose
-    additional restrictions on action bodies --- e.g., only allowing
-    straight-line code, with no conditional statements or expressions.
-
-Thus P4 programs using `if` statements within actions are likely to be less
-portable than programs that avoid doing so.
-
-As mentioned above, enhancing `p4c` would enable a larger variety of `if`
-statements within actions to be supported.
-
-* [p4c issue #644](https://github.com/p4lang/p4c/issues/644)
-* [behavioral-model issue #379](https://github.com/p4lang/behavioral-model/pull/379)
-
 
 ### Restrictions on code in the `ComputeChecksum` control