Help: Expand string(MAKE_C_IDENTIFIER) docs

The MAKE_C_IDENTIFIER subcommand was also buried in the docs for the
TIMESTAMP subcommand, so it has been pulled out to its own subheading.

Help/command/string.rst

@@ -282,6 +282,18 @@ CONFIGURE
 
 Transform a string like :command:`configure_file` transforms a file.
 
+MAKE_C_IDENTIFIER
+"""""""""""""""""
+
+::
+
+  string(MAKE_C_IDENTIFIER <input string> <output variable>)
+
+Convert each non-alphanumeric character in the ``<input string>`` to an
+underscore and store the result in the ``<output variable>``.  If the first
+character of the string is a digit, an underscore will also be prepended to
+the result.
+
 RANDOM
 """"""
 
@@ -346,13 +358,6 @@ If no explicit ``<format string>`` is given it will default to:
    %Y-%m-%dT%H:%M:%S    for local time.
    %Y-%m-%dT%H:%M:%SZ   for UTC.
 
-
-::
-
-  string(MAKE_C_IDENTIFIER <input string> <output variable>)
-
-Write a string which can be used as an identifier in C.
-
 .. note::
 
   If the ``SOURCE_DATE_EPOCH`` environment variable is set,

Help/manual/cmake-generator-expressions.7.rst

@@ -289,7 +289,8 @@ Available output expressions are:
 ``$<UPPER_CASE:...>``
   Content of ``...`` converted to upper case.
 ``$<MAKE_C_IDENTIFIER:...>``
-  Content of ``...`` converted to a C identifier.
+  Content of ``...`` converted to a C identifier.  The conversion follows the
+  same behavior as :command:`string(MAKE_C_IDENTIFIER)`.
 ``$<TARGET_OBJECTS:objLib>``
   List of objects resulting from build of ``objLib``. ``objLib`` must be an
   object of type ``OBJECT_LIBRARY``.