CXX-2723 Explicitly document throwing exception from APM callback is undefined behavior

[eramongodb]

Resolves CXX-2723. Verified by this patch.

Adds explicit documentation in the form of a warning that throwing an exception from an APM callback is undefined behavior. This requirement has always existed but was not properly documented.

As a QoI improvement, despite (now) being explicitly documented as UB, this PR proposes adding noexcept specifiers to the internal APM callback functions (the actual callback that invokes the registered user-provided callback) to reduce the scope of runtime errors should this requirement be violated, as control flow should not have returned from libmongoc via an exception to begin with (the program is already in an invalid state at this point). Any user code that currently violates the nothrow requirement will now terminate via noexcept rather than be allowed to continue with potentially unbounded libmongoc runtime errors. If this is considered too excessive (consider: our own test cases violate this requirement in failure conditions, as Catch2 uses exceptions to report test assertion failures), we can omit the addition of noexcept specifiers.

As a drive-by improvement, ran doxygen -u to update the Doxyfile to address obsolete tag warnings (TCL_SUBST, COLS_IN_ALPHA_INDEX, PERL_PATH, MSCGEN_PATH).

[vector-of-bool]

LGTM

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re: Catch2 issues: Only REQUIRE throws exceptions, and looking at the file I would guess that it should be using CHECK() or CHECKED_IF() instead of REQUIRE for every condition.

[kevinAlbs]

If this is considered too excessive (consider: our own test cases violate this requirement in failure conditions, as Catch2 uses exceptions to report test assertion failures), we can omit the addition of noexcept specifiers.

I prefer the well defined terminate to undefined behavior.