[docs] fix most podchecker issues

at least the valid ones. podchecker warns about valid
L<name|link> tags.

See podchecker docs/*.pod docs/*/*.pod (not the book)
See also GH #1053 to use Pod::Checker in the test and
GH #1054 to relax the fill-column: 100 coda to allow 78
also, to write valid pod.
runcores: fix docs, match docs with implementation and vice versa

use docs/running.pod as description for the runcore, debugging and
optimization options.
add missing docs for the runcores.

we already have a new parrot manpage for the rest.  allow the old runcores
in the implementation as described in the docs.
but disable the exec runcore, as it is different from the jit runcore.
there is now side-effect of producing a native executable (as of now).