docs(api): Document LIBTMUX_TMUX_FORMAT_SEPARATOR

Introduced initially via #343 as TMUX_SEPERATOR but later renamed
to LIBTMUX_TMUX_FORMAT_SEPARATOR.

Author: tony

docs/api.md

@@ -188,3 +188,17 @@
 ```{eval-rst}
 .. autoclass:: libtmux.test.EnvironmentVarGuard
 ```
+
+## Environmental variables
+
+### tmux format separator
+
+```{versionadded} 0.11.0b0
+
+```
+
+`LIBTMUX_TMUX_FORMAT_SEPARATOR` can be used to override the default string used
+to split `tmux(1)`'s formatting information.
+
+If you find any compatibility problems with the default, or better yet find a string copacetic
+many environments and tmux releases, note it at https://github.com/tmux-python/libtmux/discussions/355.