Merge pull request #3741 from cpcloud/read-html

TST/BUG: fix bs4 tests that were getting erroneously run when lxml is installed but not bs4
DOC: start documenting the various pitfalls of reading html
DOC: change formatting

DOC: more formatting
DOC: add bold substitutions
DOC: fill out bold links and rephrase
DOC: fill out link to gotchas
DOC: add gigantic install.rst warning