Profiling: Make better docs about what metadata APIs are legal to call in which situations

The profiler has pretty poor documentation in this area. What little that exists on David Broman's blog or MSDN is typically either overly loose (we give the profiler writer the entire IMetadatEmit API and somehow expect them to know which modifications will work?) or overly restrictive (we say do all edits during ModuleLoadFinished, but some of them actually work later too)

We likely need to bucket all the possible modifications into 4 categories:
a) Never legal
b) Legal at Module load for images that aren't precompiled
c) Legal at Module load for any image
d) Legal at any time