docs/manual: update informations about C library in internal backend

Now that we have eglibc and glibc support in the internal backend, and
no longer marked as experimental, a little bit of rewording is
needed. It is no longer necessary to indicate that uClibc was
historically supported as the only C library, and that the glibc
support is experimental. We also update the rest of the description to
be less uClibc specific.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>