[Orgmode] FILETAGS documentation clarification

[PT]

I tried FILETAGS for the first time and it failed to work and it
took a while until I found out why.

I assumed orgfiles are scanned dynamically for filetags, so I
don't have to do anything just write it into the file. Turned out I
had to reload the file as well, because org cached filetag info.

I suggest adding a sidenote about it to this page for new users,
so they know filetags don't work automatically as soon as they
are inserted into the file:

http://orgmode.org/manual/Tag-inheritance.html