docs/plugins.rst: fix signal name "page_generator_context"

The old "pages_generate_context" dates from before pelican-3.2
standardization, but AFAIK, "page_generate_context" was never correct.
Merge pull request #1119 from jedbrown/jed/fix-signal-name-doc

docs/plugins.rst: fix signal name "page_generator_context"
Fix #1117 Make intra-link support all url-value HTML attributes.
Fine-tune url-value HTML attributes list.
Merge pull request #1123 from calfzhou/intra-link

Fix #1117 Make intra-link support all url-value HTML attributes.