nvim.1: Misc. improvements (2)

Besides some minor rewording, this commit does the following:

- the argument to the `q' flag is optional, so denote it as such
- the cross references to vim (`.Xr vim 1') were pointless, just refer
  to it as `Vim'
- the `d' flag doesn't actually take any arguments; `nvim -d' works
  just fine, so make the manual reflect that
- the `L' flag is just an alias for the `r' flag, so clarify that
- there was a lot of redundancy between the `o', `O', and `p' flags,
  unify their descriptions a bit

Helped-by: Justin M. Keyes <justinkz@gmail.com>

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