Update emcc.txt from rst docs (#11098)

Result of running `make -C site text`

site/build/text/docs/tools_reference/emcc.txt

@@ -1,3 +1,4 @@
+
 Emscripten Compiler Frontend (emcc)
 ***********************************
 
@@ -119,8 +120,14 @@ Options that are modified or new in *emcc* are listed below:
    * When compiling to object files, this is the same as in *Clang*
      and *gcc*, it adds debug information to the object files.
 
-   * When linking, this is equivalent to -g3 (preserve JS whitespace
-     and compiled function names).
+   * When linking, this is equivalent to -g3.
+
+"-gseparate-dwarf[=FILENAME]"
+   Preserve debug information, but in a separate file on the side.
+   This is the same as "-g", but the main file will contain no debug
+   info, while debug info will be present in a file on the side
+   ("FILENAME" if provided, otherwise the same as the wasm file but
+   with suffix ".debug.wasm").
 
 "-g<level>"
    Controls the level of debuggability. Each level builds on the
@@ -133,8 +140,9 @@ Options that are modified or new in *emcc* are listed below:
       * "-g2": When linking, preserve function names in compiled
         code.
 
-      * "-g3": When compiling to object files, keep debug info (this
-        is the same as -g).
+      * "-g3": When compiling to object files, keep debug info,
+        including JS whitespace, function names, and LLVM debug info
+        if any (this is the same as -g).
 
       * "-g4": When linking, generate a source map using LLVM debug
         information (which must be present in object files, i.e., they
@@ -284,7 +292,21 @@ Options that are modified or new in *emcc* are listed below:
    "Module" (for example, you can define "Module['print']" there).
 
 "--post-js <file>"
-   Like *--pre-js`*, but emits a file *after* the emitted code.
+   Like "--pre-js", but emits a file *after* the emitted code.
+
+"--extern-pre-js <file>"
+   Specify a file whose contents are prepended to the JavaScript
+   output. This file is prepended to the final JavaScript output,
+   *after* all other work has been done, including optimization,
+   optional "MODULARIZE"-ation, instrumentation like "SAFE_HEAP", etc.
+   This is the same as prepending this file after "emcc" finishes
+   running, and is just a convenient way to do that. (For comparison,
+   "--pre-js" and "--post-js" optimize the code together with
+   everything else, keep it in the same scope if running *MODULARIZE*,
+   etc.).
+
+"--extern-post-js <file>"
+   Like "--extern-pre-js", but appends to the end.
 
 "--embed-file <file>"
    Specify a file (with path) to embed inside the generated
@@ -547,8 +569,9 @@ Options that are modified or new in *emcc* are listed below:
 
       * <name> **.bc** : LLVM bitcode.
 
-      * <name> **.o** : WebAssembly object file (unless fastcomp or -flto is
-        used in which case it will be in LLVM bitcode format).
+      * <name> **.o** : WebAssembly object file (unless fastcomp or
+        -flto is used in which case it will be in LLVM bitcode
+        format).
 
       * <name> **.wasm** : WebAssembly without JavaScript support
         code ("standalone wasm"; this enables "STANDALONE_WASM").