[Doc][clang] Some PGO documentation improvements.

[phosek]

LGTM

clang/docs/UsersManual.rst

 
 2. Run the instrumented executable with inputs that reflect the typical usage.
    By default, the profile data will be written to a ``default.profraw`` file
    in the current directory. You can override that default by using option
    ``-fprofile-instr-generate=`` or by setting the ``LLVM_PROFILE_FILE``
    environment variable to specify an alternate file. If non-default file name
    is specified by both the environment variable and the command line option,
    the environment variable takes precedence. The file name pattern specified
-   can include different modifiers: ``%p``, ``%h``, and ``%m``.
+   can include different modifiers: ``%p``, ``%h``, ``%m``, ``%t``, and ``%c``.
 
    Any instance of ``%p`` in that file name will be replaced by the process
    ID, so that you can easily distinguish the profile output from multiple
    runs.
 
    .. code-block:: console
 
      $ LLVM_PROFILE_FILE="code-%p.profraw" ./code
2. Run the instrumented executable with inputs that reflect the typical usage.
    profile data generated by profiler runtime. The resulting merged "raw" profile data
    file still needs to be converted to a different format expected by the compiler (
    see step 3 below).
 
    .. code-block:: console
 
      $ LLVM_PROFILE_FILE="code-%m.profraw" ./code
 
+   See `this <SourceBasedCodeCoverage.html#running-the-instrumented-program>`_ section
+   about the ``%t``, and ``%c`` modifiers.
 
 3. Combine profiles from multiple runs and convert the "raw" profile format to
    the input expected by clang. Use the ``merge`` command of the
    ``llvm-profdata`` tool to do this.
 
    .. code-block:: console
 
      $ llvm-profdata merge -output=code.profdata code-*.profraw
.. option:: -fprofile-generate[=<dirname>]
   .. code-block:: console
 
     $ llvm-profdata merge -output=code.profdata yyy/zzz/
 
   If the user wants to turn off the auto-merging feature, or simply override the
   the profile dumping path specified at command line, the environment variable
   ``LLVM_PROFILE_FILE`` can still be used to override
   the directory and filename for the profile file at runtime.
+  To override the path and filename at compile time, use
+  ``-Xclang -fprofile-instrument-path=/path/to/file_pattern.profraw``.
 
 .. option:: -fcs-profile-generate[=<dirname>]
 
   The ``-fcs-profile-generate`` and ``-fcs-profile-generate=`` flags will use
   the same instrumentation method, and generate the same profile as in the
   ``-fprofile-generate`` and ``-fprofile-generate=`` flags. The difference is
   that the instrumentation is performed after inlining so that the resulted
   profile has a better context sensitive information. They cannot be used