Help: Better explain CMAKE_<LANG>_EXTENSIONS_DEFAULT
Improve it to better indicate that this represents the compiler's
default and mustn't be modified by the user. Clarify when it's used as
the default and added a reference to CMAKE_<LANG>_EXTENSIONS for when
the user wants to set the default for new targets.
Additionally, add explanations for the default initialization by
CMAKE_<LANG>_EXTENSIONS_DEFAULT to all <LANG>_EXTENSIONS pages and
references to CMP0128. Slightly reduce the wordiness of the default
initialization explanations by removing an unnecessary "it is".
Fixes: #22828 (closed)
Backport: release
Edited by Brad King