[docs] [clang] Small documentation change for compilation databases

[croepha]

Summary

We have an page dedicated to compliation databases including
various ways to generate them, but we don't mention that clang
has a built in method to do this. This addresses that.

[tschuett]

https://sarcasm.github.io/notes/dev/compilation-database.html#clang

[joerg]

clang/docs/JSONCompilationDatabase.rst

33

I think you mean "concatenate", but otherwise LGTM?

[croepha]

Fixed spelling

[croepha]

clang/docs/JSONCompilationDatabase.rst

33

Yes, thank you

[croepha]

Hey, @joerg I don't have commit access, would you mind landing this for me?

clang/docs/JSONCompilationDatabase.rst

-  Build systems are inherently sequential in the build graph, for
    independently of the build still need the generated source code to
    exist, running tools multiple times over unchanging source does not
    require serialization of the runs according to the build dependency
    graph.
 
 Supported Systems
 =================
 
+Clang has the ablity to generate compilation database fragments via
+the :option:`-MJ argument <clang -MJ\<arg>>`. You can concatenate those
+fragments together between ``[`` and ``]`` to create a compilation database.
+
 Currently `CMake <https://cmake.org>`_ (since 2.8.5) supports generation
 of compilation databases for Unix Makefile builds (Ninja builds in the
 works) with the option ``CMAKE_EXPORT_COMPILE_COMMANDS``.
 
 For projects on Linux, there is an alternative to intercept compiler
 calls with a tool called `Bear <https://github.com/rizsotto/Bear>`_.
 
 `Bazel <https://bazel.build>`_ can export a compilation database via