Developer guide: Recommend raw strings for all docstrings

src/doc/en/developer/coding_basics.rst

@@ -667,6 +667,9 @@ You are strongly encouraged to:
 
 - Use LaTeX typesetting (see :ref:`section-latex-typeset`).
 
+- Use raw strings (``r"""..."""``), regardless of whether the docstring
+  currently contains any backslashes or not.
+
 - Liberally describe what the examples do.
 
   .. NOTE::
@@ -779,8 +782,7 @@ In Sage's documentation LaTeX code is allowed and is marked with **backticks**:
     ```x^2 + y^2 = 1``` yields `x^2 + y^2 = 1`.
 
 **Backslashes:** For LaTeX commands containing backslashes, either use double
-backslashes or begin the docstring with a ``r"""`` instead of ``"""``. Both of
-the following are valid::
+backslashes or begin the docstring with a ``r"""`` instead of ``"""``::
 
     def cos(x):
         """
@@ -792,6 +794,8 @@ the following are valid::
         Return `\sin(x)`.
         """
 
+We strongly suggest to use the latter.
+
 **MATH block:** This is similar to the LaTeX syntax ``\[<math expression>\]``
 (or ``$$<math expression>$$``). For instance: