ENH: Improve CMP0012 doc and message formatting

This fixes the CMP0012 description to have a one-line summary in the
'brief' section and the rest of the explanation in the 'full' section.
It makes the warning message shorter and improves formatting of the
policy documentation, especially in the HTML pages.  The convention is
already used by all other policies.

Source/cmPolicies.cxx

@@ -358,12 +358,13 @@ cmPolicies::cmPolicies()
 
     this->DefinePolicy(
     CMP0012, "CMP0012",
+    "The if() command can recognize named boolean constants.",
     "In CMake versions prior to 2.6.5 the only boolean constants were 0 "
     "and 1. Other boolean constants such as true, false, yes, no, "
     "on, off, y, n, notfound, ignore (all case insensitive) were recognized "
     "in some cases but not all. In later versions of cmake these values are "
     "treated as boolean constants more consistently and should not be used "
-    "as variable names. Please do not use them as variable names.",
+    "as variable names. "
     "The OLD behavior for this policy is to allow variables to have names "
     "such as true and to dereference them. "
     "The NEW behavior for this policy is to treat strings like true as a "