Change 'charse_error' to 'charset_error' in load_files.

'charset_error' is the name used in the documentation; I changed the
function to match it. It checks and deprecates the old name.
Revise documentation about handling text and bytes.
Add a documentation section about decoding text.

This is a stage that has to happen before extracting features. Adding
this documentation helps address a bug report about UnicodeDecodeErrors.
Move the new "Decoding text files" doc section

It should come after the other section that describes text feature
extraction.