Improve documentation of `ly:note-extra-source-file`
The documentation of ly:note-extra-source-file
says
Register a file, e.g., an image file, as being needed to compile the current file. This is used for the
-dembed-source-code
option. A parser may optionally be specified.
This sounds like it creates a Makefile dependency file or something similar. Instead, however, the function actually makes LilyPond embed an arbitrary file into the output PDF, which can't be deduced IMHO from this description.