Also update command specifications to match descriptions.

GitHub now nicely generates a documentation site for us at
http://brew.sh/brew based on our docs folder. Optimise the output of
this and the GitHub docs directory for readability and the various user
groupings.

Addressing a few spots where I looked for terminology and did not find what I was looking for. Input based on a recent [podcast with Mike](https://changelog.com/podcast/223)

Closes #1040.

Always a good idea to quote command substitution.

Also, explicitly welcome new/beginner contributors and give them a suggested
starting point to contribute to Homebrew. Based on the boiler plate I tend to
send people to for GSoC, Outreachy or generally people who say "I want to try
and contribute to open-source".

* Include --desc in brew search help text.
* Update brew search --desc text in README.
* Remove mention of alias in brew search help text to avoid any
  confusion.

Closes Homebrew/homebrew#49500.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>