Advice docs are too verbose

[allan2]

The docs in the advice module are long. They are mostly copied and pasted from the Linux man page.

Some current problems:

• There are unlinked references to man pages such as fork(2).
• The formatting is copied over as well, such as two spaces after a sentence period.
• The sheer length makes it hard to read advice.rs.
• The information is not always up to date. MADV_HUGEPAGE works with file-mapped memory since Linux 5.4 #120

I propose limiting each value's comment to just a few lines. A brief explanation and a safety block when needed. The Advice enum's comment already includes a link to the man page, for those who need more explanation.

madvise is also not just for Linux. Other reference pages include FreeBSD and Mac OS X.

#133 affected

[de-vri-es]

Agreed. If we can simplify the text here and refer to external documentation for platform specific details, I think that's better.