OK, recent simple example:

Google query: git display file at revision. Immediate answer (without even having to click any links, it's in the result description): `git show revision:file`

Total time: 5 seconds

Trying to reproduce with man and help:

  man git

search for display, finds nothing

start scrolling down

notice git-show (show various types of objects); sounds like a likely candidate

  git show <revision> <file>

..no output

  git show -h

  usage: git log [<options>] [<since>..<until>] [[--] <path>...]
     or: git show [options] <object>...

.. useful

  man git show
  man git-show

OPTIONS <object>... The names of objects to show. For a more complete list of ways to spell object names, see "SPECIFYING REVISIONS" section in git-rev-parse(1).

  man git-rev-parse

a lot about specifying revisions, nothing about how actually specify a file

Give up. Google it.