Clarify make build -j double red herring

During the migration to 3.13 it was suggested to use `make build -j` to
automatically speed up the building process of the documentation to use
all available cores. This is red herring in two different ways:

 * `make`'s `-j` flag can be used to parallelise the processing of
   `make` target dependencies instead of magically apply parallelisation
   to the underlying commands. In our case the `make build` command has
   a sequential sequence of dependencies (venv -> setup -> do_build) so
   no actual parallelisation is achieved when passing `-j`.
   `sphinx-build` can be parallelised by giving *it* a `-j` flag,
   which we already do (`-j auto`). In other words, there's nothing to
   do on our side to speed things up even further. What we *do* have
   though is a `make` `SPHINX_JOBS` variable, which acts as the argument
   to `sphinx-build -j`. Using this users can reduce the number of CPUs
   they want to use for the build process.

 * Secondly, an empty `make -j` option (i.e., without a number
   afterwards), contrary to popular belief, runs as many recipes in
   parallel as possible [1], *with no regards to the number of available
   cores*. In most scenarios this is fine because the number of
   dependencies for a given target isn't that great, or because the
   target instructions are not that heavy, but it can generate
   difficult-to-understand problems on occasion. For example, if you are
   building a shared library that consists on 250 files of heavy C++,
   you'll end up spawning 250 C++ compilers at the same time, which
   surely will consume all the memory of your computer, causing them to
   be OOM-killed. Bottom-line is: never use -j without a value.

Signed-off-by: Rodrigo Tobar <[email protected]>

Author: rtobar

.overrides/upgrade-python-version.rst

@@ -48,9 +48,11 @@ We are currently in branch 3.12, and we want to update the strings from 3.13.
 
    .. note::
 
-      You can also run `make build -j` to use more than 1 core (but keep in mind
-      this is not always faster).
+      The underlying ``sphinx-build`` command uses all available cores by default.
+      Use the ``SPHINX_JOBS`` ``make`` variable (defaults to ``auto``)
+      to specify an explicit amount, e.g. ``make build SPHINX_JOBS=10``.
 
+   .. note::
       It may fail the build because there may be files
       that don't exist anymore in the new branch.
       If that's the case, just continue with the steps