(bibtex-format-entry): Use `bibtex-empty-field-re' only on the text of

(bibtex-format-entry): Use `bibtex-empty-field-re' only on the text of
fields, not on entire field lines.
(bibtex-autofill-entry): Use `bibtex-empty-field-re' on a string,
not on part of a buffer.
(bibtex-empty-field-re): Don't match nonempty field text strings like
"{letters\\macro{}more letters}".  Clarify docstring.
(bibtex-sort-entry-class, bibtex-autokey-titleword-ignore)
(bibtex-entry-offset, bibtex-parse-association)
(bibtex-parse-field-name): Fix typos in docstrings.
(bibtex-field-list, bibtex-find-crossref): Fix typos in error
messages.