Help: Document CMAKE_FIND_APPBUNDLE and CMAKE_FIND_FRAMEWORK

Add dedicated documents for these and link to them instead of
duplicating their documentation in every find command.

Help/variable/CMAKE_FIND_APPBUNDLE.rst

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+CMAKE_FIND_APPBUNDLE
+--------------------
+
+This variable affects how ``find_*`` commands choose between
+OS X Application Bundles and unix-style package components.
+
+On Darwin or systems supporting OS X Application Bundles, the
+``CMAKE_FIND_APPBUNDLE`` variable can be set to empty or
+one of the following:
+
+``FIRST``
+  Try to find application bundles before standard programs.
+  This is the default on Darwin.
+
+``LAST``
+  Try to find application bundles after standard programs.
+
+``ONLY``
+  Only try to find application bundles.
+
+``NEVER``
+  Never try to find application bundles.

Help/variable/CMAKE_FIND_FRAMEWORK.rst

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+CMAKE_FIND_FRAMEWORK
+--------------------
+
+This variable affects how ``find_*`` commands choose between
+OS X Frameworks and unix-style package components.
+
+On Darwin or systems supporting OS X Frameworks, the
+``CMAKE_FIND_FRAMEWORK`` variable can be set to empty or
+one of the following:
+
+``FIRST``
+  Try to find frameworks before standard libraries or headers.
+  This is the default on Darwin.
+
+``LAST``
+  Try to find frameworks after standard libraries or headers.
+
+``ONLY``
+  Only try to find frameworks.
+
+``NEVER``
+  Never try to find frameworks.