Use `X` to toggle the showing of terminal buffers.

When the feature to toggle showing terminal buffers was introduced, a
mapping was added for `H` to perform this toggling in the BufExplorer
window; however, the `H` key was immediately mapped again by
BufExplorer's logic to use `:keepjumps` with some movement keys (`H`,
`M`, `L`, etc.), thus disabling the intended `H` mapping:

  ```vim
  nnoremap <script> <silent> <nowait> <buffer> H             :call <SID>ToggleShowTerminal()<CR>

  for k in ["G", "n", "N", "L", "M", "H"]
      execute "nnoremap <buffer> <silent>" k ":keepjumps normal!" k."<CR>"
  endfor
  ```

As these movement keys are useful for navigating the BufExplorer window,
use `X` instead of `H` to toggle the showing of terminal buffers, as `X`
is normally an editing feature (for deleting characters before the
cursor) that's not needed in the BufExplorer window.

In addition, the `X` command has been documented in `bufexplorer.txt`
and in BufExplorer's `<F1>` help text.

doc/bufexplorer.txt

@@ -103,6 +103,7 @@ Commands to use once exploring:
  u             Toggles the showing of "unlisted" buffers.
  V             Open the selected buffer in another window on the left of the current.
  v             Open the selected buffer in another window on the right of the current.
+ X             Toggles the showing of terminal buffers.
 
 Once invoked, Buffer Explorer displays a sorted list (MRU is the default
 sort method) of all the buffers that are currently opened. You are then

plugin/bufexplorer.vim

@@ -714,7 +714,7 @@ function! s:MapKeys()
     nnoremap <script> <silent> <nowait> <buffer> u             :call <SID>ToggleShowUnlisted()<CR>
     nnoremap <script> <silent> <nowait> <buffer> v             :call <SID>SelectBuffer("split", "vr")<CR>
     nnoremap <script> <silent> <nowait> <buffer> V             :call <SID>SelectBuffer("split", "vl")<CR>
-    nnoremap <script> <silent> <nowait> <buffer> H             :call <SID>ToggleShowTerminal()<CR>
+    nnoremap <script> <silent> <nowait> <buffer> X             :call <SID>ToggleShowTerminal()<CR>
 
 
     for k in ["G", "n", "N", "L", "M", "H"]
@@ -839,6 +839,7 @@ function! s:CreateHelp()
         call add(header, '" u : toggle showing unlisted buffers')
         call add(header, '" V : open buffer in another window on the left of the current')
         call add(header, '" v : open buffer in another window on the right of the current')
+        call add(header, '" X : toggle showing terminal buffers')
     else
         call add(header, '" Press <F1> for Help')
     endif