MMShareFindPboard is supposed to prevent MacVim from updating the system
find pasteboard when searching, but the previous behavior was that it
would only set the Vim-specific format to the find pasteboard so you
could still share search patterns across Vim windows but it has the
effect of clearing the global find pasteboard. Just fix that by making
MacVim not touch the system find pasteboard at all when
MMShareFindPboard is set to NO to respect user's wish. Cmd-G will still
use the system find pasteboard to make it easy to search in another
macOS program and jump to Vim. The user can always use `n` instead if
that's not desired.

Also update docs to make this clear, and rename all "OS X" to "macOS" to
keep up with the times.

Close macvim-dev#773

Cmd-E (Edit -> Use Selection for Find") in most macOS apps follow the
convention that it shares to the system Find Pasteboard, while Cmd-G
uses the Find Pasteboard to find the next item. Make Cmd-E always do
that even if MMShareFindPboard is set to NO. This way, normal searches
in Vim wouldn't pollute the system find pasteboard, but could still have
a way to share to it by using this action.

Vim patch 8.1.560

Targets macOS 10.8+

Features:
- macOS Mojave (10.14) is now supported.
    - MacVim's UI now works with Dark Mode.
    - Fixed broken rendering and flickering under Mojave when using the
      default Core Text renderer. macvim-dev#757
- guioption 'k' is supported again. macvim-dev#731
    - This option prevents window from resizing when UI elements such as
      toolbars or tabs show or hide themselves.

Fixes:
- Fixed misc fullscreen and window resizing bugs and artifacts macvim-dev#745
- Dragging tabs to reorder now works properly macvim-dev#789
- Fixed timer callback handling in GUI macvim-dev#749
- Fixed native tabs (10.12+) interring with Vim tabs macvim-dev#788
- Fixed Japanese IME Ctrl-U/Ctrl-O handling macvim-dev#742
- Fixed MMShareFindPboard and Cmd-E/Cmd-G interactions macvim-dev#780
- Better handling of guifontwide font size macvim-dev#737
- Better python discovery in default vimrc macvim-dev#739

Known Issues:
- Scrolling performance is slightly worse under Mojave macvim-dev#796

Script interfaces have compatibility with these versions:
- Lua 5.3
- Perl 5.18
- Python2 2.7
- Python3 3.7
- Ruby 2.5

Vim patch 8.1.560

Targets macOS 10.8+

Features:
- macOS Mojave (10.14) is now supported.
    - MacVim's UI now works with Dark Mode.
    - Fixed broken rendering and flickering under Mojave when using the
      default Core Text renderer. #757
- guioption 'k' is supported again. #731
    - This option prevents window from resizing when UI elements such as
      toolbars or tabs show or hide themselves.

Fixes:
- Fixed misc fullscreen and window resizing bugs and artifacts #745
- Dragging tabs to reorder now works properly #789
- Fixed timer callback handling in GUI #749
- Fixed native tabs (10.12+) interring with Vim tabs #788
- Fixed Japanese IME Ctrl-U/Ctrl-O handling #742
- Fixed MMShareFindPboard and Cmd-E/Cmd-G interactions #780
- Better handling of guifontwide font size #737
- Better python discovery in default vimrc #739

Known Issues:
- Scrolling performance is slightly worse under Mojave #796

Script interfaces have compatibility with these versions:
- Lua 5.3
- Perl 5.18
- Python2 2.7
- Python3 3.7
- Ruby 2.5