Help: Add section headers to find_package command

The document is pretty long but is already mostly organized into
sections.  Add explicit section headers.

Help/command/find_package.rst

@@ -7,6 +7,9 @@ find_package
 
 Find an external project, and load its settings.
 
+Basic Signature and Module Mode
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 ::
 
   find_package(<PackageName> [version] [EXACT] [QUIET] [MODULE]
@@ -42,6 +45,9 @@ package-by-package basis (details below).
 See the :command:`cmake_policy` command documentation for discussion
 of the ``NO_POLICY_SCOPE`` option.
 
+Full Signature and Config Mode
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 User code should generally look for packages using the above simple
 signature.  The remainder of this command documentation specifies the
 full command signature and details of the search process.  Project
@@ -114,6 +120,9 @@ fatal error is generated and the configure step stops executing.  If
 ``<PackageName>_DIR`` has been set to a directory not containing a
 configuration file CMake will ignore it and search from scratch.
 
+Version Selection
+^^^^^^^^^^^^^^^^^
+
 When the ``[version]`` argument is given Config mode will only find a
 version of the package that claims compatibility with the requested
 version (format is ``major[.minor[.patch[.tweak]]]``).  If the ``EXACT``
@@ -206,6 +215,9 @@ package configuration files are encouraged to name and install them
 such that the procedure outlined below will find them without
 requiring use of additional options.
 
+Search Procedure
+^^^^^^^^^^^^^^^^
+
 CMake constructs a set of possible installation prefixes for the
 package.  Under each prefix several directories are searched for a
 configuration file.  The tables below show the directories searched.
@@ -346,6 +358,9 @@ enabled.
 Every non-REQUIRED ``find_package`` call can be disabled by setting the
 :variable:`CMAKE_DISABLE_FIND_PACKAGE_<PackageName>` variable to ``TRUE``.
 
+Package File Interface Variables
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 When loading a find module or package configuration file ``find_package``
 defines variables to provide information about the call arguments (and
 restores their original state before returning):