ls-remote doc: explain what each example does

ls-remote doc: explain what each example does

While it's good to have several examples to solidify the output pattern
and generally demonstrate how to use the command, most other EXAMPLES
sections (e.g., git-show-branch.txt, git-remote.txt) additionally
describe the problem/situation to which the example is applicable.

Follow this example in the ls-remote documentation.

Signed-off-by: Sean Allred <allred.sean@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>