Improve doc string of eshell and eshell-buffer-name

* lisp/eshell/eshell.el (eshell-buffer-name, eshell): Improve doc
strings, by clarifying how buffers are named.  Also refer users to the
Info manual.  (Bug#37178)
1 job for master in 90 minutes and 1 second (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #3903
test-all

01:30:01

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds