Change terminology in docs from "environment variables" -> "shell variables"

Hi - A coworker recently introduced me to fish-shell and I am loving it compared to tcsh/bash/zsh.  I am especially happy about the new vim command-line editing support.   :)

However, in reading the docs, I was a bit confused by repeated references to "environment variables" when I believe it would be more accurate to use the phrase "shell variables".  In common usage since at least 1979, an environment variable is one that the O.S. will pass onto child processes created by the current process (see http://man7.org/linux/man-pages/man7/environ.7.html and http://en.wikipedia.org/wiki/Environment_variable).

For most of the docs, the variables referenced are either not inheritable or not necessarily inheritable by child processes.  As such, the most accurate term in this case is "shell variable".  The term "environment variable" should be reserved for variables create by "set -x" in fish-shell (i.e. "export" in bash/zsh or "setenv" in csh/tcsh).

I made some minor edits to the doc files, changing the term "environment variable" -> "shell variable" when appropriate, and keeping and/or emphasizine the term "environment variable" for those var's created using "set -x".  I think this will help to avoid confusion for new users and match the historical distinction between shell & environment variables.

Alan Thompson