Help: Update description of the -D option to CMake

Fixes #18021

Help/manual/OPTIONS_BUILD.txt

@@ -11,7 +11,7 @@
  cache-format file.
 
 ``-D <var>:<type>=<value>, -D <var>=<value>``
- Create a cmake cache entry.
+ Create or update a cmake cache entry.
 
  When cmake is first run in an empty build tree, it creates a
  CMakeCache.txt file and populates it with customizable settings for

Source/cmake.h

@@ -550,7 +550,7 @@ 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 or update a cmake cache entry." }, \
     { "-U <globbing_expr>", "Remove matching entries from CMake cache." },    \
     { "-G <generator-name>", "Specify a build system generator." },           \
     { "-T <toolset-name>",                                                    \