If you want the header file to be found when you place it in the libraries directory you need to place it in a folder of the same name without the '.h'.
c:\users\mike\documents\arduino\libraries\myheader\myheader.h
This is incorrect. While it is common practice to do this, it is not required.
The IDE searches many places and directories within those places looking for headers.
bundled libraries, core area, sketchbook libraries, to list a few.
Once it finds the header file, it then creates a -I option to the compiler to add the directory where the header file is found to the compiler search path.
The issue that can arise is when there are collisions. (multiple directories with the same header file name)
Using a directory of the same name as the header will give the directory priority when the header file is found in multiple places.
--- bill