Help: Spell '-D' option consistently across documentation (#15575)

Always show the option with its argument separate in summary text.
State in the main documentation that the option and its argument can be
given together too.

Help/manual/OPTIONS_BUILD.txt

@@ -27,6 +27,9 @@
  command in the project sets the type to ``PATH`` or ``FILEPATH``
  then the ``<value>`` will be converted to an absolute path.
 
+ This option may also be given as a single argument:
+ ``-D<var>:<type>=<value>`` or ``-D<var>=<value>``.
+
 ``-U <globbing_expr>``
  Remove matching entries from CMake cache.
 

Help/manual/cmake.1.rst

@@ -9,7 +9,7 @@ Synopsis
 .. parsed-literal::
 
  cmake [<options>] (<path-to-source> | <path-to-existing-build>)
- cmake [(-D<var>=<value>)...] -P <cmake-script-file>
+ cmake [(-D <var>=<value>)...] -P <cmake-script-file>
  cmake --build <dir> [<options>] [-- <build-tool-options>...]
  cmake -E <command> [<options>...]
  cmake --find-package <options>...

Source/cmake.h

@@ -403,7 +403,7 @@ private:
 
 #define CMAKE_STANDARD_OPTIONS_TABLE \
   {"-C <initial-cache>", "Pre-load a script to populate the cache."}, \
-  {"-D <var>:<type>=<value>", "Create a cmake cache entry."}, \
+  {"-D <var>[:<type>]=<value>", "Create a cmake cache entry."}, \
   {"-U <globbing_expr>", "Remove matching entries from CMake cache."}, \
   {"-G <generator-name>", "Specify a build system generator."},\
   {"-T <toolset-name>", "Specify toolset name if supported by generator."}, \