fsck_msdosfs.8: document -M

[delphij]

Summary

Document -M.

[bcr]

Good to go!

[emaste]

Might we remove it in the future? I.e., should we add a note that users should not rely on it remaining?

[mckusick]

This looks good.

[mckusick]

In D23049#505425, @emaste wrote:

Might we remove it in the future? I.e., should we add a note that users should not rely on it remaining?

If it is part of the test scripts, then it will need to remain.

I believe that it might be useful for embedded systems where it will allow a tighter bound on memory usage than if mmap is used.