Document --closure-args in emcc reference. (#15683)

The environment variable `EMCC_CLOSURE_ARGS` does not work well in
all places (e.g. Bazel), so it would be good document the ability to use flags
to impact Closure.

docs/emcc.txt

@@ -253,6 +253,16 @@ Options that are modified or new in *emcc* are listed below:
      * Closure is only run if JavaScript opts are being done ("-O2" or
        above).
 
+"--closure-args=<args>"
+   [link] Pass arguments to the *Closure compiler*. This is an
+   alternative to "EMCC_CLOSURE_ARGS".
+
+   For example, one might want to pass an externs file to avoid
+   minifying JS functions defined in "--pre-js" or "--post-js" files.
+   To pass to Closure the "externs.js" file containing those public
+   APIs that should not be minified, one would add the flag: "--
+   closure-args=--externs=path/to/externs.js"
+
 "--pre-js <file>"
    [link] Specify a file whose contents are added before the emitted
    code and optimized together with it. Note that this might not

site/source/docs/tools_reference/emcc.rst

@@ -232,6 +232,13 @@ Options that are modified or new in *emcc* are listed below:
     - If closure compiler hits an out-of-memory, try adjusting ``JAVA_HEAP_SIZE`` in the environment (for example, to 4096m for 4GB).
     - Closure is only run if JavaScript opts are being done (``-O2`` or above).
 
+``--closure-args=<args>``
+   [link]
+   Pass arguments to the :term:`Closure compiler`. This is an alternative to ``EMCC_CLOSURE_ARGS``.
+
+   For example, one might want to pass an externs file to avoid minifying JS functions defined in ``--pre-js`` or ``--post-js`` files.
+   To pass to Closure the ``externs.js`` file containing those public APIs that should not be minified, one would add the flag:
+   ``--closure-args=--externs=path/to/externs.js``
 
 .. _emcc-pre-js: