[clang-tidy][NFC] Clarify documentation for misc-definitions-in-headers

Add information about what fixes are provided by
the check, and how to enable them.

Issue: #55093

Author: PiotrZSL

clang-tools-extra/docs/clang-tidy/checks/misc/definitions-in-headers.rst

@@ -88,6 +88,11 @@ from multiple translation units.
    template <class T>
    constexpr T pi = T(3.1415926L);
 
+When :program:`clang-tidy` is invoked with the `--fix-notes` option, this check
+provides fixes that automatically add the ``inline`` keyword to discovered
+functions. Please note that the addition of the ``inline`` keyword to variables
+is not currently supported by this check.
+
 Options
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