Clarify documentation and message for CMP0012

This commit re-words the warning message produced for CMP0012 to avoid
the word 'you' since often the person reading the message is not the
author of the code.  We also add an example of the bad OLD behavior to
the policy documentation.

Source/cmIfCommand.cxx

@@ -269,10 +269,9 @@ namespace
       if (cmSystemTools::IsOff(def))
         {
         cmPolicies* policies = makefile->GetPolicies();
-        errorString = "You have used a variable or argument named \""
+        errorString = "A variable or argument named \""
           + newArg
-          + "\" in a conditional statement. Please be aware of issues "
-          + "related to policy CMP0012. "
+          + "\" appears in a conditional statement.  "
           + policies->GetPolicyWarning(cmPolicies::CMP0012);
         status = cmake::AUTHOR_WARNING;
         }
@@ -285,10 +284,9 @@ namespace
       if (!cmSystemTools::IsOff(def))
         {
         cmPolicies* policies = makefile->GetPolicies();
-        errorString = "You have used a variable or argument named \""
+        errorString = "A variable or argument named \""
           + newArg
-          + "\" in a conditional statement. Please be aware of issues "
-          + "related to policy CMP0012. "
+          + "\" appears in a conditional statement.  "
           + policies->GetPolicyWarning(cmPolicies::CMP0012);
         status = cmake::AUTHOR_WARNING;
         }

Source/cmPolicies.cxx

@@ -362,7 +362,9 @@ cmPolicies::cmPolicies()
     "In CMake versions 2.6.4 and lower 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 "
+    "in some cases but not all.  "
+    "For example, the code \"if(TRUE)\" might have evaluated as false.  "
+    "In later versions of cmake these values are "
     "treated as boolean constants more consistently and should not be used "
     "as variable names. "
     "The OLD behavior for this policy is to allow variables to have names "