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
MFC r251631: illumos #3742 zfs comments need cleaner, more consistent style

- Make more of ZFS's comments use a natural English writing flow.
- Break up long paragraphs, fix various typos and spelling errors.
- Don't prefix a function description with its name when the function
  definition immediately follows.
- Remove useless comments.
- Add extra whitespace where it makes the comments more readable.

New comments were separated from this change and added in r251629.

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