doc/dev: use prompt directive when appropriate

doc/dev: use prompt directive when appropriate

for 2 reasons:

* sphinx renders codeblock using python syntax highlighting by default,
  so it's not surprising that it highlight keywords like "export" in
  command line samples. so to render command line code blocks, we'd
  better specify the syntax explicitly for better rendering result.
* with the help of "prompt" directive, user is able to copy and paste
  the command without the prompt. for instance, with the default
  "::" directive, user will copy "$ ceph df", which is not very
  convenient, but with "prompt" directive, user only copies
  "ceph df".

Signed-off-by: Kefu Chai <kchai@redhat.com>