[AC21.4] Improve glyph documentation

RECOMMEND 21.4 >>>>> "vin" == Vin Shelton <acs(a)xemacs.org&gt; writes: vin> This mostly does apply, except for one hunk about background pixmap. vin> I'm always on the lookout for improvements to the glyph documentaion. I found out why it doesn't apply for me---I'd already done some work and reorganization in 21.4. Many of the ideas ended up in the 21.5 patch, but there's more work to be done there. Here's the 21.4 patch. Sorry about the long delay. This patch is against release-21-4 (== 21.4.17 for these files according to the ChangeLog). I recommend it, but maybe you want to apply, format, and look at it for a while before committing. There are a couple of ickinesses I don't have time to think about how to fix right now (eg, the node on images doesn't contain any text, only a menu), and there are surely typos. If you have specific requests, I'll be happy to address them. Keeping my fingers crossed that the lstream bug won't eat it! The patch should be 3241 lines long.