[doc, 4/n] invoke.texi: there is no part 4

[Sandra Loosemore]

Part 4 of this series was originally intended to move the section "Using 
Precompiled Headers" from invoke.texi to extend.texi.  But, as I started 
to dig around to decide exactly where its should go, I didn't see a good 
place to put it there, either -- extend.texi is still very disorganized. 
 I've decided to leave the section where it is for now.

In the long run, I think maybe it should be grouped with other 
documentation describing preprocessor extensions, since it is 
essentially describing an extension to #include.

-Sandra