Editing an .fgd is pretty easy with Notepad as Kol said... but beware, not following the format will cause parsing problems. Most Mods are based on the Original HL .fgd and this has been modified several times. Removing entity information is Ok, as it narrows down the amout of options you have (Take CS-expert for example, they removed func_push and gravity because they couldn't work out where you would ever use it) But adding entity specific stuff means that the dll must have been modified. You can't just add an entity to the .fgd for it to work.
Not sure where you would get specific information about this but try ModBD or Rust or a site that deals with Coding. And ofcourse there is always Collective.