The correct possessive with it is "its", not "it's".

Closes: politza#689

- Add / Fix docstrings for functions.
- Make punctuation changes as suggested by `checkdoc`

This commit makes changes in a subset of files of `pdf-tools`. Some of
the major files are still pending changes, and will be addressed in a
later commit.

Partially Fixes: #64

- Remove :group arguments from `defcustom` declarations
  - `defcustom` picks up `defgroup` names from the same file, so
    specifying the group is not necessary
- Ensure that all function-names are properly quoted using #'
- Add an explicit header-option setting lexical-binding to t
- Ensure that cl-check-type uses `satisfies` for filenames
- Replace all `defadvice` with `advice-add`
- Replace `(list ...)` patterns with `\`(...)` patterns

Fixes: #62
Fixes: #32
Partially Fixes: #24

This removes the positial arguemnt triplet from the define-minor-mode
calls, because they are obsolete since Emacs<21. See the following
paragraph in the help section of define-minor-mode.

For backward compatibility with the Emacs<21 calling convention, the
keywords of define-minor-mode can also be preceded by the obsolete
triplet INIT-VALUE LIGHTER KEYMAP.

Fixes the long-standing issue of displaying pdf contents
in proper resolution on retina Macs using vanilla (not a fork) emacs
[1], [2]

This commit squashes changes contributed by @iostapyshyn. It
introduces the following changes:

- Use `frame-scale-factor` to detect HiDPI displays in preference to
  other methods. `frame-scale-factor` is a new function introduced in
  Vanilla Emacs to return the scale factor of the backing store.
- Allow scaling for `png` type images as well (if
  `pdf-view-use-scaling` is `t`), since this works well and I'm not
  sure why it was originally disabled.

1: politza/pdf-tools#51
2: hlissner/doom-emacs#4989

Closes #13

`t` is not a valid keymap.  I looked through the other minor-mode
definitions and didn't see any other such uses.

Found that this was causing me problems, see Wilfred/helpful#114.