DOC: Update 1.8.0 release notes.

Order highlights by significance. This is a subjective judgement.
Mention the linalg enhancements and illustrate with an example.
Update memmap.py

Clarify `memmap` docstring to better reflect the meaning of *offset*.
Merge pull request #3886 from charris/update-1.8.0-notes

DOC: Update 1.8.0 release notes.
Merge pull request #3890 from andreas-h/patch-1

Update memmap.py