improve match explanations

Using == and != proved too hard to read, so went with [TRUE] and [FALSE]
after the term. I would kind of liked to have used emojis for a green
check and red X, but probably that is too fancy to be a good idea.

Make --explain output be inside [ ] with whitespace around them, to
avoid it ending with eg "[FALSE]]" and to make it easier to visually
find the start of it.

Sponsored-by: Dartmouth College's DANDI project

doc/todo/option_to_explain/comment_2_71616074104ab70eb76ad5edb9207b5c._comment

@@ -0,0 +1,12 @@
+[[!comment format=mdwn
+ username="joey"
+ subject="""comment 2"""
+ date="2023-07-26T19:31:40Z"
+ content="""
+--explain is implemented and expanations for preferred content expressions,
+annex.largefiles, and annex.addunlocked are now displayed.
+
+Leaving this open because I do think that explanations of numcopies and
+content presence checking should be added, as well as perhaps explanations
+of drop proofs.
+"""]]