Initial updates to magics re: `%load_ext`, etc.

Also copied over preamble from octavemagic doc regarding installation of
appropriate packages to other magics, and a note about installing R or Octave
separately where relevant.
Fixed formatting errors in rmagic R() docstring

Things now mostly render correctly in both terminal and HTML versions of the
docs.

Note that there are still numerous warnings regarding these magics. At least
some of them appear to be from the debauched manner in which the docs are being
generated from the @magic_arguments. In addition to the warnings you get when
building the docs, if you look at the HTML source for, e.g., rmagic, you'll see
that the options are formatted in numerous ways. I get a "--" converted to an
en-dash (&#8211;) for example.

In case you're looking at this in a system that messes up the formatting of the
HTML code-point above, the en-dash is 8211.
Merge pull request #4209 from davclark/magic-doc-fixes

Magic doc fixes