Introduce disable= as a better way of disabling a build

Prefixing commit ID with ! and a message will still work, but that's
very silly. Using disable= is now the correct way.

docs/fdroid.texi

@@ -746,21 +746,23 @@ The above specifies to build version 0.12, which has a version code of 3.
 The third parameter specifies the tag, commit or revision number from
 which to build it in the source repository.
 
-If the commit version starts with a !, that version is not built. Instead,
-everything after the ! is used as a reason why it can't be built. The
-purpose of this feature is to allow non-buildable releases (e.g. the source
-is not published) to be flagged, so the scripts don't generate repeated
-messages about them. (And also to record the information for review later).
-If an apk has already been built, ! causes it to be deleted once 
-@code{fdroid update} is run; this is the procedure if ever a version has to 
-be replaced.
-
 In addition to the three, always required, parameters described above,
 further parameters can be added (in name=value format) to apply further
 configuration to the build. These are (roughly in order of application):
 
 @table @code
 
+@item disable=<message>
+Disables this build, giving a reason why. (For backwards compatibility, this
+can also be achieved by starting the commit ID with '!')
+
+The purpose of this feature is to allow non-buildable releases (e.g. the source
+is not published) to be flagged, so the scripts don't generate repeated
+messages about them. (And also to record the information for review later).
+If an apk has already been built, disabling causes it to be deleted once 
+@code{fdroid update} is run; this is the procedure if ever a version has to 
+be replaced.
+
 @item subdir=<path>
 Specifies to build from a subdirectory of the checked out source code.
 Normally this directory is changed to before building,