advice: Document that they all default to true

By definition, the default value of "advice.*" variables must be true and
they all control various additional help messages that are designed to aid
new users. Setting one to false is to tell Git that the user understands
the nature of the error and does not need the additional verbose help
message.

Also fix the asciidoc markup for linkgit:git-checkout[1] in the
description of the detachedHead advice by removing an excess colon.
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -115,35 +115,32 @@ in the appropriate manual page. You will find a description of non-core
 porcelain configuration variables in the respective porcelain documentation.
 
 advice.*::
-	When set to 'true', display the given optional help message.
-	When set to 'false', do not display. The configuration variables
-	are:
+	These variables control various optional help messages designed to
+	aid new users. All 'advice.*' variables default to 'true', and you
+	can tell Git that you do not need help by setting these to 'false':
 +
 --
 	pushNonFastForward::
 		Advice shown when linkgit:git-push[1] refuses
-		non-fast-forward refs. Default: true.
+		non-fast-forward refs.
 	statusHints::
 		Directions on how to stage/unstage/add shown in the
 		output of linkgit:git-status[1] and the template shown
-		when writing commit messages. Default: true.
+		when writing commit messages.
 	commitBeforeMerge::
 		Advice shown when linkgit:git-merge[1] refuses to
 		merge to avoid overwriting local changes.
-		Default: true.
 	resolveConflict::
 		Advices shown by various commands when conflicts
 		prevent the operation from being performed.
-		Default: true.
 	implicitIdentity::
 		Advice on how to set your identity configuration when
 		your information is guessed from the system username and
-		domain name. Default: true.
-
+		domain name.
 	detachedHead::
-		Advice shown when you used linkgit::git-checkout[1] to
+		Advice shown when you used linkgit:git-checkout[1] to
 		move to the detach HEAD state, to instruct how to create
-		a local branch after the fact.  Default: true.
+		a local branch after the fact.
 --
 
 core.fileMode::