Ok to backport Eshell manual improvements to 29?

[Jim Porter]

I just pushed a branch - "scratch/backport-eshell-docs-to-29" - that 
backports all the relevant changes to the Eshell manual that are 
currently on master. I've double-checked that the documentation all 
looks right to me (specifically that the 29 branch doesn't mention any 
features only present on master), but before I merge to the 29 branch, I 
wanted to give others the chance to look over the changes too.

I've opted not to squash any of the commits and to add a note to each 
commit message indicating the Git SHA for each master commit. Hopefully 
that will make it easier if anyone needs to look back on these commits 
in the future.