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/git-annex-common-options.mdwn

@@ -37,6 +37,13 @@ Most of these options are accepted by all git-annex commands.
   what to do. The explanations will be inside square brackets.
   For example, "[foo is not present here]"
 
+  This includes explanations of why preferred content expressions and
+  other similar expressions like annex.largefiles match or fail to match.
+  In these explanations, the expression is displayed, with each term
+  followed by "[TRUE]" or "[FALSE]" to indicate the value.
+  Irrelevant terms will be ommitted from the explanation, for example 
+  `"exclude=* and copies=1"` will be displayed as `"exclude=*[FALSE]"`
+
 * `--debug`
 
   Display debug messages to standard error.

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.
+"""]]