Used during Emacs initialization to scan the `etc/DOC...' file.
This searches the `etc/DOC...' file for doc strings and
records them in function and variable definitions.
The function takes one argument, FILENAME, a string;
it specifies the file name (without a directory) of the DOC file.
That file is found in `../etc' now; later, when the dumped Emacs is run,
the same file name is found in the `doc-directory'. (fn FILENAME)