xmllint: various fixes (#1450)
xmllint: various fixes (#1450)

- fix string example, and improve its description
- move xpath information to the main description
- add "tags" synonym for "nodes"
- add a html example
- simplify description of doctype example
- capitalize "xml" consistently
deekim/tldr,UncommonPleb/tldr,cptdeka10/tldr,UncommonPleb/tldr,cptdeka10/tldr,deekim/tldr,katekyan/tldr,felixonmars/tldr,zoisom/tldr,UncommonPleb/tldr,zoisom/tldr,felixonmars/tldr,zoisom/tldr,slash3b/tldr,katekyan/tldr,katekyan/tldr,deekim/tldr,cptdeka10/tldr,slash3b/tldr,felixonmars/tldr,Phizho/tldr,Phizho/tldr,Phizho/tldr,slash3b/tldr