x86_energy_perf_policy: Document MSRHEADER

Add comprehensive comments documenting the MSRHEADER macro usage and
expected build system configuration.

The MSRHEADER macro is not defined in the source file and must be
provided by the build system (typically as
-DMSRHEADER=\"msr-index.h\" or similar). This can be confusing for
developers building the code independently.

Adding documentation clarifies:
- That MSRHEADER is a macro expected to be defined externally
- What types of headers are typically used
- Example build flags showing how to define it

This prevents build failures and confusion when compiling without
the proper build system context.

Fixes: 4beec1d75196 ("tools/power x86_energy_perf_policy: support HWP.EPP")
Assisted-by: GitHub-Copilot:Claude-Sonnet-4.6
Signed-off-by: Len Brown <len.brown@intel.com>

0 files changed, 0 insertions, 0 deletions