advice: make all entries stylistically consistent

advice: make all entries stylistically consistent

In general, rewrite entries to the following form:

1. Clause or sentence describing when the advice is shown
2. Optional “to <verb>” clause which says what the advice is
   about (e.g. for resetNoRefresh: tell the user that they can use
   `--no-refresh`)

Concretely:

1. Use “shown” instead of “advice shown”
   • “advice” is implied and a bit repetitive
2. Use “when” instead of “if”
3. Lead with “Shown when” and end the entry with the effect it has,
   where applicable
4. Use “the user” instead of “a user” or “you”
5. implicitIdentity: rewrite description in order to lead with *when*
   the advice is shown (see point (3))
6. Prefer the present tense (with the exception of pushNonFFMatching)
7. waitingForEditor: give example of relevance in this new context
8. pushUpdateRejected: exception to the above principles

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>