doc: join default pre-commit paragraphs

doc: join default pre-commit paragraphs

Join two paragraphs that start with the standard “The default <hook>,
when enabled” into one and put it at the end of the “pre-commit”
section.

The trailing whitespace paragraph was added in the first commit for the
doc, in 6d35cc76 (Document hooks., 2005-09-02). Then 3e14dd2c (mention
use of "hooks.allownonascii" in "man githooks", 2019-02-20) updated the
“pre-commit” section to mention the non-ASCII check that was added in
d00e364d.[1] But this paragraph was added one-past the original
“default” paragraph, after the env. variable paragraph, and starts
exactly the same. That causes the flow of this section to feel
off (paragraphs in order):

1. Invoked by <cmd> and what parameters it takes
2. The default 'pre-commit' hook catches introduction of trailing
   whitespace
3. `GIT_EDITOR=:`
4. The default pre-commit' hook catches introduction of non-ASCII
   filenames

Let’s instead join these two paragrahs and explain the whole behavior of
the default script.

† 1: Extend sample pre-commit hook to check for non ascii filenames,
     2009-05-19

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