Clarify rose_arch source documentation.

A literal reading of current ```rose_arch``` documentation suggests that source paths are always taken as relative: 

> Paths are assumed to be relative to $ROSE_SUITE_DIR or to $ROSE_SUITE_DIR/PREFIX if source-prefix=PREFIX is specified.

A user here took this too literally and deliberately chopped the front off an absolute source path, but **not left a leading /** and wondered why his source files weren't found.

This change clarifies the documentation.