doc: various updates

* Don't use 'smart typography' when generating man page
* Document PATTERN and PATH
* Capitalise place-holder names consistently
* Add note about PATH overriding glob/ignore rules
* Update args.rs for new PATH capitalisation

Fixes #725

doc/convert-to-man

@@ -1,5 +1,5 @@
 #!/bin/sh -e
 
-pandoc -s -t man rg.1.md -o rg.1
+pandoc -s -f markdown-smart -t man rg.1.md -o rg.1
 sed -i.bak 's/\.TH.*/.TH "rg" "1"/g' rg.1
 rm -f rg.1.bak # BSD `sed` requires the creation of a back-up file