[vim/vim] runtime(doc): document &t_8u as &t_8f and &t_8b are documented (PR #20837)

Closes #20413

`terminal-output-codes` are out of my depth, but I understand at least this much.

There are 40-odd `terminal-output-codes` defined by Vim, but only the three starting with `t_8` have a default value (for me).

    &t_8f = "\<Esc>[38;2;%lu;%lu;%lum"  # foreground
    &t_8b = "\<Esc>[48;2;%lu;%lu;%lum"  # background
    &t_8u = "\<Esc>[58;2;%lu;%lu;%lum"  # underline

The first two (foreground and background) are mentioned at `:h xterm-true-color`. I found the rest under a table in `:h termcap-options`.

I've updated all three values to the colon form in my vimrc.

    &t_8f = "\<Esc>[38:2::%lu:%lu:%lum"  # foreground
    &t_8b = "\<Esc>[48:2::%lu:%lu:%lum"  # background
    &t_8u = "\<Esc>[58:2::%lu:%lu:%lum"  # underline

After several years using Vim in Windows Terminal, I can't see a difference from setting the two (foreground and background) mentioned specifically at `:h xterm-true-color`.

However, the `&t_8u` underline value (only present in the termcap-options table) causes problems with `set spell` and the `yegappan/lsp` plugin in Windows Terminal. I've added the underline form to the documentation.

You can view, comment on, or merge this pull request online at:

  https://github.com/vim/vim/pull/20837

-- Commit Summary --

  * runtime(doc): document &t_8u as &t_8f and &t_8b are documented

-- File Changes --

    M runtime/doc/term.txt (10)

-- Patch Links --

https://github.com/vim/vim/pull/20837.patch
https://github.com/vim/vim/pull/20837.diff

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