Add Filesystem Documentation to User's Guide with limitations

Summary

The RTEMS User's Guide needs a filesystems section that describes the limitations of the various filesystems RTEMS supports in their current state.

JFFS2

The JFFS2 implementation in RTEMS supports operation on NOR and NAND, but does not yet support Erase Block Summary (EBS) on NAND which affects mount times for large, full filesystems. JFFS2 has a maximum filesystem size of 4GB-1 block. JFFS2 uses 32-bit time in the on-disk format and is not Y2038-safe. JFFS2 read/write performance degrade when full.

MSDOS

MSDOS uses 32-bit time in the on-disk format and is not Y2038-safe. The maximum file size is 4GB. There are limits on the accuracy of timestamps (1s granularity). Symlinks are not supported.