[perl #72892] Perl should support PERLmmnnLIB environment variable

This patch to perlrun.pod expands the documentation of the directory
hierarchy searched under the locations specified by PERL5LIB.  It now
explicitly mentions the version- and architecture-specific directories
added, and also notes that $Config{inc_version_list} directories are
added.  These additions were suggested by Damon Atkins.