standardize the recommended upgrade command

in both docs and error messages.

Recent bug reports indicate that `cleanup` commands are essential
in some situations.
don't use Markdown-style backtick in error msgs

Because backtick has a different meaning in the shell, and the user
may copy-and-paste such text.

Some light re-wording included as well, and a reference to "brew cask edit"
changed to "brew cask cat" for simplicity.
switch to `diskutil` for eject

Mostly to see if this reduces random Travis errors, but also
because `diskutil eject` is a documented interface, and
`hdiutil eject` is not.
Merge pull request #4969 from rolandwalker/doc_standard_upgrade_command

standardize the recommended upgrade command
Merge pull request #4970 from rolandwalker/no_backtick_in_error_messages

don't use Markdown-style backtick in error msgs
Annotatate unexpected $LOAD_PATH in doctor

Warning if first element is not Homebrew-cask's own lib dir
Merge pull request #4965 from rolandwalker/doctor_load_path_annotation

Annotatate unexpected $LOAD_PATH in doctor
Added WD Security
Merge pull request #4947 from callumacrae/wd-security

Added WD Security
Merge pull request #4975 from rolandwalker/diskutil_eject

switch to `diskutil` for eject
DSL: add `tags` stanza

only limited/defined keys are permitted
- :developer

forward-compatibility: no associated functionality is
provided.  Intentionally left undocumented.

References: #4688