documentation: expose search data packing options.

documentation: expose search data packing options.

There are new SEARCH_RESULT_ID_BYTES, SEARCH_FILE_OFFSET_BYTES and
SEARCH_NAME_SIZE_BYTES options exposed in both Doxygen and Python doc
generator. Because they affect some internals that the regular user
shouldn't need to care about and don't reflect anything tangible like
project symbol count, they're not really documented.

On the other hand, they can't really be estimated beforehand The
estimator would either overestimate, leading to files larger than they
could be, or it underestimates, leading to random corner cases that
would be impossible to track down. Thus the project just starts with the
small sizes and if they're not enough, an exception with a (hopefully
helpful-enough) message is raised, suggesting users to adjust their
config.