Docs: Add caution about ranges like py{39-314}

Tox's ultra-concise environment naming creates some challenges when
using features like range expansions. If the user tries to define
a range like `py{39-314}`, they'll get a lot more than they bargained
for. Add a "Caution" box to the documentation about the range feature,
reminding users to consider what their ranges mean _numerically_,
rather than semantically.

Signed-off-by: FeRD (Frank Dana) <[email protected]>

Author: ferdnyc

docs/config.rst

@@ -1688,6 +1688,13 @@ will create the following envs:
 
 Negative ranges will also be expanded (``{3-1}`` -> ``{3,2,1}``), however, open ranges such as ``{1-}``, ``{-2}``, ``{a-}``, and ``{-b}`` will not be expanded.
 
+.. caution::
+
+  Be conscious of the number of significant digits in your range endpoints.
+  A range like ``py{39-314}`` will not do what you may expect.
+  (It expands to 275 environment names,
+  ``py39``, ``py40``, ``py41`` … ``py314``.)
+  Instead, use ``py3{9-14}``.
 
 
 Generative section names