docs/manual/ccache-support.txt: expand explanation about ccache cache location

The manual explanation seemed to imply that the cache is always in
~/.buildroot-ccache/, but it's just the default value. Clarify this
point.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>