doc: Clarify how to use the '#:python' parameter.

* doc/guix.texi (Build Systems): Clearly distinguish between the package that
provides the Python interpreter for the build and the package that is to be
built.  Also, mention why one might want to use a specific Python version.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

diff --git a/doc/guix.texi b/doc/guix.texi
index 1a7f188..7a27ab3 100644 (file)
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -2854,8 +2854,11 @@ For packages that install stand-alone Python programs under @code{bin/},
 it takes care of wrapping these programs so that their @code{PYTHONPATH}
 environment variable points to all the Python libraries they depend on.
 
-Which Python package is used can be specified with the @code{#:python}
-parameter.
+Which Python package is used to perform the build can be specified with
+the @code{#:python} parameter.  This is a useful way to force a package
+to be built for a specific version of the Python interpreter, which
+might be necessary if the package is only compatible with a single
+interpreter version.
 @end defvr
 
 @defvr {Scheme Variable} perl-build-system