Need better doc of compilation mode

[Drew Adams]

1. I just discovered this feature (yes, I think it's a feature) by accident:
Click (mouse-2) a grep hit on the file name or line number, and the text
that matches the regexp is highlighted in the source buffer. Click instead
anywhere else on the grep hit and the whole line is highlighted in the
source buffer. Nice.

I see no documentation for this. The logical place to document it would be
in the Compilation Mode node of the Emacs manual.

2. Node Compilation Mode says that the source hit is only momentarily
highlighted. The word "momentarily" should be removed - that is just the
default behavior. It also says "You can change the behavior of this
highlighting with the variable `next-error-highlight'." It should instead
say that that variable determines whether and for how long the hit is
highlighted.

3. I filed a bug separately about the default highlight period
(`next-error-highligt') being ridiculously short - it is 0.1 sec, so short I
never even see the highlighting; it should be perhaps 10 or 30 seconds. I
personally think the default value should be `t', which just moves the
highlight each time you use `next-error'.

4. Node Grep Searching does not even say that what is said about
`compilation-mode' in node Compilation Mode applies also to grep. It should
say that buffer *grep* is in Grep mode, which has Compilation mode as a
parent mode. More importantly, it should contain a link to node Compilation
Mode, so readers can learn how to use Grep mode! We should never assume that
users read the manual linearly, front to back. If a user goes to the manual
looking for information on `grep', then s?he should be able to easily get to
all pertinent information about it.