This commit adds some documentation on what are the limitations of using
Picolibc as a standard C library for native modules.
This also contains a reference to the "errno" issue when building
natmods on RV32 that the PR this commit is part of, as it is not obvious
how to approach this issue when encountered for the first time.
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>
e993f53877