Maybe a combination of private fork with comments and separate markdown files with notes (maybe in the same private fork)
Consider using special "symbols" in comments like "MYDOCS_XXX" that you search for in your modified version of the code base, and refer to in other places. These will survive renames of function names etc by the upstream authors.
Consider using special "symbols" in comments like "MYDOCS_XXX" that you search for in your modified version of the code base, and refer to in other places. These will survive renames of function names etc by the upstream authors.