Update terminfo notes

One of my colleagues at work just spent a frustrating period trying to
get italics working with iTerm etc, based partly on these instructions.
The missing links were:

  - missing TERMINFO variable in the environment
  - using an old/unpatched version of vim-solarized-colors

This commit tries to fix the latter a little bit by adding some
documentation to these files.

Signed-off-by: Wincent Colaiuta <win@wincent.com>