[clang-tidy] Improve diagnostic message for misc-definitions-in-header.

Summary:
Users might get confused easily when they see the check's message on
full template function speciliations.

Add a note to the output message, which mentions these kind of function
specializations are treated as regular functions.

Reviewers: alexfh

Reviewed By: alexfh

Subscribers: JDevlieghere, cfe-commits

Differential Revision: https://reviews.llvm.org/D29928

llvm-svn: 295048