Help: Revise and format policy CMP0025 and CMP0047 docs

Add inline reST markup as appropriate.  Word CMP0047 docs more like
those of CMP0025.  State explicitly that the policies must be set
before the project or enable_language command calls.
Do not warn by default when policy CMP0025 or CMP0047 is not set

These policies are triggered by the use of a particular compiler rather
than outdated CMake code in a project.  Avoid warning in every project
that enables a language by not displaying the policy warning by default.
Add variable CMAKE_POLICY_WARNING_CMP<NNNN> to control the warning
explicitly.  Enable the warning with --debug-output, --trace, or -Wdev.

This breaks with strict policy convention because it does not provide
developers with any warning about the behavior change by default.
Existing projects will continue to build without a warning or change in
behavior.  When a developer changes the minimum required version of
CMake in a project to a sufficiently high value (3.0), the project will
suddenly get the new compiler id and may break, but at least the
breakage comes with a change to the project rather than the version of
CMake used to build it.

Breaking strict policy convention is worthwhile in this case because
very few projects will be affected by the behavior change but every
project would have to see the warning if it were enabled by default.
Merge branch 'master' into revise-compiler-id-policies
cmMakefile: Port PolicyOptionalWarningEnabled to string APIs

This method was added in a commit based on a release branch that
pre-dated the std::string API changes.  Port the implementation to use
the string APIs instead of c_str().
Merge topic 'revise-compiler-id-policies' into next

25ebaeda cmMakefile: Port PolicyOptionalWarningEnabled to string APIs
3c76cdd9 Merge branch 'master' into revise-compiler-id-policies
9f9fcf9e Do not warn by default when policy CMP0025 or CMP0047 is not set
9f7fb645 Help: Revise and format policy CMP0025 and CMP0047 docs