Help: Clarify --debug-trycompile usage with try_compile

Explain that try_compile calls need to be protected with conditions to
avoid running more than once so that it is easy to isolate a single
try_compile to perform with --debug-trycompile.
Merge topic 'doc-try_compile-debug-advice' into next

abbe91c5 Help: Clarify --debug-trycompile usage with try_compile