DOC: fixed correlate docstring.

Replaced correlation formula with corrected version that matches the
numpy implementation. Added comment on possible further different
definitions of correlation. Added examples.
DOC: replaced typeset formulas by pure ASCII equivalents.

Replaced typeset formulas by pure ASCII equivalents. Should now be
easier to read in source files.
Merge pull request #3913 from bspinnler/fix_correlate_docstring

DOC: fixed correlate docstring.