Update documentation for named's -M option

Update documentation for named's -M option

Remove "external" from the list of legal values for the -M command-line
option as it has not been allowed since the internal memory allocator
was removed by commit 55ace5d3aa4cb8c1b70a94679e1dd6d716dfd8bb.

Make the style of the relevant paragraph more in line with the next one
and split its contents up into an unordered list of options for improved
readability.