I don't find `man git` to be bad at all. Git is complex, and its man pages do the right thing by referring to sources for basics info like giteveryday, and referring to in-depth guides as well. Individual man pages are also pretty good, see `man git-rebase`. It starts with an overall explanation of what rebase does, with examples, and then covers configuration options and flags. It's a lot of stuff, but it's pretty good as far as documentation goes.
GNU packages often have documentation that's bad in the typical "Linux docs are bad" way. Try `man less`. First, it commits a grave sin in having a totally useless one-line summary, which reads "less - opposite of more". Funny, but totally useless and doesn't even remotely suggest what the command does (if you know what more is on UNIX, you surely know what less does). Or `man grep`. It's a reference page, very good for knowing what all the options do, but with no useful everyday examples, and with gems like these:
> Finally, certain named classes of characters are predefined within bracket expressions, as follows. Their names are self explanatory, and they are [:alnum:], [:alpha:], [:cntrl:], [:digit:], [:graph:], [:lower:], [:print:], [:punct:], [:space:], [:upper:], and [:xdigit:].
Self explanatory? Yes, if you used grep in the 90s. Is alnum alphanumeric or all numbers? Is alpha short for alphabet, as in what most people would intuitively call letters? What's xdigit? Extra digits? Except digits? Oh, it's hex digits. Pretty obvious that periods and commas are in punct... but also + * - {} are punct, among other stuff.
`man tar` is extremely comprehensive, an impressive reference, but very hard to figure out if you've never used tar.
I've been recently looking at FreeBSD documentation for common commands, and the source code as well. Both are so much better than the usual GNU versions you find on Linux.