Use XTVERSION for terminal-specific workarounds

As mentioned in earlier commit
("Query terminal before reading config").

Closes #11812

doc_src/language.rst

@@ -1693,6 +1693,10 @@ Fish also provides additional information through the values of certain environm
 
    the process ID (PID) of the shell.
 
+.. envvar:: fish_terminal
+
+   the name and version of the terminal fish is running inside (for example as reported via :ref:`XTVERSION <term-compat-xtversion>`).
+
 .. envvar:: history
 
    a list containing the last commands that were entered.

doc_src/terminal-compatibility.rst

@@ -224,9 +224,12 @@ Optional Commands
      - Ss
      - Set cursor style (DECSCUSR); Ps is 2, 4 or 6 for block, underscore or line shape.
      - VT520
-   * - ``\e[ Ps q``
+   * - .. _term-compat-xtversion:
+
+       ``\e[ Ps q``
      - n/a
      - Request terminal name and version (XTVERSION).
+       This is only used for temporary workarounds for incompatible terminals.
      - XTerm
    * - ``\e[?25h``
      - cvvis