Improve look of method headings in HTML

Source Code Pro needs to be bold to stand out against Lato properly.

Remove whitespace between method headings, it looks silly spread out all
over the page.
Space out lines in a paragraph
Fix rb_file_const() when used outside file.c

Previously rdoc assumed rb_file_const was called within file.c, but this
is not true in trunk.  This caused the FNM_* constants to go missing.

Now these constants appear in File::Constants as expected.

(The constants do not appear in File even though File::Constants is
included there, this is a limitation of other parts of RDoc.)