Add MAIN_THREAD_EM_ASM_PTR to docs and changelog. NFC (#20284)

Author: sbc100

ChangeLog.md

@@ -22,6 +22,8 @@ See docs/process.md for more on how version tagging works.
 -----------------------
 - `EM_JS` and `EM_ASM` that are present in side module now have their syntax
   validated at build time. (#20258)
+- `MAIN_THREAD_EM_ASM_PTR` macro added for code that returns a pointer.  This
+  mirrors the existing `EM_ASM_PTR`. (#20261)
 
 3.1.46 - 09/15/23
 -----------------
@@ -49,7 +51,8 @@ See docs/process.md for more on how version tagging works.
 - A new settings `TABLE_BASE` was introduced that can be used to place static
   function addresses (table slots) at a certain offset.  This defaults to 1
   which is the previously fixed value. (#20149)
-- Clang's error detection of unused variables `-Wunused-variable` improved, which may require changes in user code (see #20169).
+- Clang's error detection of unused variables `-Wunused-variable` improved,
+  which may require changes in user code (see #20169).
 
 3.1.45 - 08/23/23
 -----------------

site/source/docs/api_reference/emscripten.h.rst

@@ -219,6 +219,10 @@ Defines
 
   Similar to :c:macro:`MAIN_THREAD_EM_ASM` but returns a ``double`` value.
 
+.. c:macro:: MAIN_THREAD_EM_ASM_PTR(code, ...)
+
+  Similar to :c:macro:`MAIN_THREAD_EM_ASM` but returns a pointer value.
+
 .. c:macro:: MAIN_THREAD_ASYNC_EM_ASM(code, ...)
 
   Similar to :c:macro:`MAIN_THREAD_EM_ASM` but is proxied in an