Fix emacs native compilation warnings

[sdwolfz]

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To reproduce, have emacs built with native compilation and notice the compilation logs. You can then open the offending file and run M-x emacs-lisp-native-compile-and-load before and after the changes to see the warning is removed.

ace-window.el:95:2: Warning: custom-declare-variable `aw-minibuffer-flag'
    docstring wider than 80 characters
ace-window.el:99:2: Warning: custom-declare-variable `aw-ignored-buffers'
    docstring wider than 80 characters
ace-window.el:104:2: Warning: custom-declare-variable `aw-ignore-on' docstring
    wider than 80 characters
ace-window.el:117:12: Warning: defcustom for ‘aw-leading-char-style’ has
    syntactically odd type ‘'(choice (const :tag single char 'char) (const
    :tag full path 'path))’
ace-window.el:138:2: Warning: custom-declare-variable `aw-frame-offset'
    docstring wider than 80 characters
ace-window.el:143:2: Warning: custom-declare-variable `aw-frame-size'
    docstring wider than 80 characters
ace-window.el:149:12: Warning: defcustom for ‘aw-char-position’ has
    syntactically odd type ‘'(choice (const :tag top left corner only
    'top-left) (const :tag both left corners 'left))’
ace-window.el:157:2: Warning: defvar `aw-dispatch-alist' docstring wider than
    80 characters
ace-window.el:191:2: Warning: custom-declare-variable `aw-make-frame-char'
    docstring wider than 80 characters
ace-window.el:449:2: Warning: docstring wider than 80 characters

[sellout]

Please merge #221, then this PR.