The mspgcc manual doesn't really describe the GNU tools. Why?

Extensive documentation, in multiple formats and languages, exists for the GNU tools. mspgcc is a port of those tools, and has all their standard features, plus a few special ones. Documenting all the standard things would be a waste of time, and probably confusing to those familiar with the GNU tools for other targets. The mspgcc documentation only seeks to document things which are special to mspgcc.