[NFC][analyzer] Update docs of CodeChecker --ctu-ast-mode (#144901)

The documentation of the cross translation unit analysis mentioned a
certain flag of `CodeChecker` (an external open source tool that can be
used to drive the static analysis), but the information about it was
obsolete: apparently the name of the flag, the names of the possible
values, and the default value were all changed.

Currently `CodeChecker analyze --help` displays this flag as
```
--ctu-ast-mode {load-from-pch,parse-on-demand}
  Choose the way ASTs are loaded during CTU analysis. Only available if
  CTU mode is enabled. Mode 'load-from-pch' generates PCH format
  serialized ASTs during the 'collect' phase. Mode 'parse-on-demand'
  only generates the invocations needed to parse the ASTs. Mode 'load-
  from-pch' can use significant disk-space for the serialized ASTs,
  while mode 'parse-on-demand' can incur some runtime CPU overhead in
  the second phase of the analysis. (default: parse-on-demand)
```
and I tried to follow this in the commands that I adjusted.

Note that this documentation file probably contains other obsolete
details as well, but I didn't try to find or fix them.

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