Doc fixes.

Doc fixes.
(previous-matching-history-element, next-history-element):
Change to work correctly even if the point is in the prompt.
(choose-completion-string): Likewise.
(minibuffer-prompt-width): New function (compatibility with old subr).
(line-move): Supply new ESCAPE-FROM-EDGE argument to
constrain-to-field.
(line-move): Use constrain-to-field to avoid
moving into a prompt.
(previous-complete-history-element): Clarify doc string.
(next-complete-history-element): Likewise.
Use field-beginning instead of point-min.
(next-matching-history-element): Use field-beginning and
erase-field instead of point-min and erase-buffer.
(previous-matching-history-element): Get the
minibuffer input with field-string instead of buffer-string.
(choose-completion-string): Likewise.
(next-history-element): Likewise.
Erase the minibuffer input with erase-field, not erase-buffer.
Use field-beginning to find the beginning of the input.