doc: reword "config file" to "configuration file"

While we do use the abbreviation "config" elsewhere in the docs, the
phrase "configuration file" is always written out in full, except in
this location in phases.txt.

--HG--
branch : stable
doc: make it easier to read how to enable extensions

We tell people all the time that enabling extensions is not a scary
thing to do, but we don't make it easy enough for an absolute novice
to do so. When they see a suggestion to do "hg extfoo bar", the error
message tells them "see hg help extensions", but that help page
doesn't actually tell them where configuration files are.

Furthermore, the big warning about why extensions aren't enabled by
default should be pushed down a little bit. Most of the extensions
shipped by hg are not all that scary, and some very basic and useful
cosmetic extensions like graphlog, color, pager, and progress, should
be enabled for many hg users.

--HG--
branch : stable
commit: amending with --close-branch (issue3445)

You can't close a branch that hasn't got a head.
newbranch + commit --close-branch must fail
newbranch + commit + commit --amend --close-branch must fail

You must not be allowed to close a branch that is not defined.

--HG--
branch : stable