Added more comments to manifests and quoted paths

An upcoming feature for SALVE will be to insist that non-keyword, non-number
symbols are handled as quoted strings, in order to give good syntax
highlighting and clarify semantics in some sticky situations.

Bad cases for the current rules include blocks like

file { source source target file }

in which the specific filenames match known keywords and potentially add
confusion to a manifest. These should be quoted anyway so that the rules for
quoting are always clear -- space-based parsing shouldn't conflict with a
string with spaces &c.