Как использовать командный символ vim, чтобы найти объяснение в его файле справки

Используйте :h select-mode, и вы увидите описание режима выбора. Здесь выдержка:

==============================================================================
8. Select mode                                          *Select* *Select-mode*

Select mode looks like Visual mode, but the commands accepted are quite
different.  This resembles the selection mode in Microsoft Windows.
When the 'showmode' option is set, "-- SELECT --" is shown in the last line.

Entering Select mode:
- Using the mouse to select an area, and 'selectmode' contains "mouse".
  'mouse' must also contain a flag for the current mode.
- Using a non-printable movement command, with the Shift key pressed, and
  'selectmode' contains "key".  For example: <S-Left> and <S-End>.  'keymodel'
  must also contain "startsel".
- Using "v", "V" or CTRL-V command, and 'selectmode' contains "cmd".
- Using "gh", "gH" or "g_CTRL-H" command in Normal mode.
- From Visual mode, press CTRL-G.                       *v_CTRL-G*

...
                                                        *gh*

gh                      Start Select mode, characterwise.  This is like "v",
                        but starts Select mode instead of Visual mode.
                        Mnemonic: "get highlighted".
...