Explain that this represents the compiler's default and mustn't be modified
by the user. Clarify when it's used as the default.

Additionally:
* Add a reference to it in cmake-compile-features in text explaining the
  feature.
* 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.