Eshell manual is (hopefully) complete!

[Jim Porter]

I've just merged to master (75278855f4a) some additions to the Eshell 
manual to try and fill in the last of the missing documentation. 
Hopefully I haven't missed anything.

To that end, could people take a quick look over the latest version of 
the Eshell manual to make sure it seems reasonably complete? There's a 
lot more detail to various sections since Emacs 28, and a fair bit since 
29 too. I've spent quite a while looking at the manual though, so it 
would be helpful to have some fresh eyes take a look as well.

If anyone finds any major gaps, I'll try to get that fixed (or to help 
get patches merged if you'd like to write the documentation yourself).

- Jim