Small fix for GTK documentation

[Ulric Eriksson <ulric edu stockholm se>]

Quoting from gtk.texi:

8<------

@strong{Note:} GTK assumes that the first field in a structure will be
placed by the compiler at the start of the structure. This is certainly
true for gcc, however, from my precursory reading of the C standard I
was unable to come to a definite conclusion as to whether this was
required or simply done for simplicity. I'm not too worried about this
assumption, though, as every C compiler I've ever encountered would work
with GTK.

8<------

Since GTK is in this case not at odds with the language it is written in,
it would be good to have this note removed or replaced with one which
explains that C guarantees that the above assumption is correct.

Ulric