man/ostree.repo-config: improve min-free-space-* docs

man/ostree.repo-config: improve min-free-space-* docs

0 is not a valid value (the units are required), and 1MB is taken to
mean 1048576 bytes, not 1000000 bytes (unlike other human-readable sizes
in libostree, which are formatted by g_format_size(), and hence use
power-of-10 units). It's probably a bit late to change this latter
point, but the documentation should mention it.

Define 'MB' etc. more precisely; include the byte counts in the
examples; and improve the formatting of the min-free-space-* paragraphs.

Signed-off-by: Will Thompson <wjt@endlessm.com>
Closes: #1706
Approved by: mwleeds