Help: Document the CMAKE_EXPORT_COMPILE_COMMANDS variable

Help/variable/CMAKE_EXPORT_COMPILE_COMMANDS.rst

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+CMAKE_EXPORT_COMPILE_COMMANDS
+-----------------------------
+
+Enable/Disable output of compile commands during generation.
+
+If enabled, generates a ``compile_commands.json`` file containing the exact
+compiler calls for all translation units of the project in machine-readable
+form.  The format of the JSON file looks like:
+
+.. code-block:: javascript
+
+  [
+    {
+      "directory": "/home/user/development/project",
+      "command": "/usr/bin/c++ ... -c ../foo/foo.cc",
+      "file": "../foo/foo.cc"
+    },
+
+    ...
+
+    {
+      "directory": "/home/user/development/project",
+      "command": "/usr/bin/c++ ... -c ../foo/bar.cc",
+      "file": "../foo/bar.cc"
+    }
+  ]
+
+.. note::
+  This option is implemented only by :ref:`Makefile Generators`
+  and the :generator:`Ninja`.  It is ignored on other generators.