work around clang bug 35333: don't crash when switching git branches

See https://bugs.llvm.org/show_bug.cgi?id=35333 for details and
explanation.

We need to do two things to work around this bug: disable -Wdocumentation,
and only use the plain text from the comments instead of the parsed
version.

REVIEW: https://phabricator.kde.org/D8857
BUG: 358205
BUG: 372686
FIXED-IN: 5.2.1