MFC r251629: 3741 zfs needs better comments

MFC r251629: 3741 zfs needs better comments

Embellish the comments in various components of ZFS.  Move some comments
around closer to what they describe.  Specifically, answer the questions:

- What are some of the edge cases of the dbuf state machine?
- What does a txg quiesce do?
- When does the DMU notify threads waiting on txg's that they may
  proceed?
- How do the calculations for RAIDZ map allocations work?
- What process do the RAIDZ I/O start and done callbacks follow?

While here, adjust the function prototype of dmu_zfetch.c:dmu_zfetch_colinear()
to match its comment which describes its return as a boolean.

Submitted by:	asomers, gibbs, will
Reviewed by:	Matthew Ahrens <mahrens@delphix.com>,
		Eric Schrock <eric.schrock@delphix.com>,
		Christopher Siden <christopher.siden@delphix.com>
Sponsored by:	Spectra Logic

git-svn-id: svn://svn.freebsd.org/base/stable/9@252749 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f