doc: clarify "config --bool" behaviour with empty string

`git config --bool xxx.yyy` returns `true` for `[xxx]yyy` but
`false` for `[xxx]yyy=` or `[xxx]yyy=""`.  This is tested in
t1300-repo-config.sh since 09bc098.

Signed-off-by: Andreas Heiduk <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: asheiduk

Documentation/config.txt

@@ -133,15 +133,15 @@ boolean::
        synonyms are accepted for 'true' and 'false'; these are all
        case-insensitive.
 
-       true;; Boolean true can be spelled as `yes`, `on`, `true`,
-		or `1`.  Also, a variable defined without `= <value>`
+	true;; Boolean true literals are `yes`, `on`, `true`,
+		and `1`.  Also, a variable defined without `= <value>`
 		is taken as true.
 
-       false;; Boolean false can be spelled as `no`, `off`,
-		`false`, or `0`.
+	false;; Boolean false literals are `no`, `off`, `false`,
+		`0` and the empty string.
 +
 When converting value to the canonical form using `--bool` type
-specifier; 'git config' will ensure that the output is "true" or
+specifier, 'git config' will ensure that the output is "true" or
 "false" (spelled in lowercase).
 
 integer::

Documentation/git.txt

@@ -571,7 +571,8 @@ example the following invocations are equivalent:
 Note that omitting the `=` in `git -c foo.bar ...` is allowed and sets
 `foo.bar` to the boolean true value (just like `[foo]bar` would in a
 config file). Including the equals but with an empty value (like `git -c
-foo.bar= ...`) sets `foo.bar` to the empty string.
+foo.bar= ...`) sets `foo.bar` to the empty string which ` git config
+--bool` will convert to `false`.
 
 --exec-path[=<path>]::
 	Path to wherever your core Git programs are installed.