(Minibuffer): Simplify introduction.

(Minibuffer): Simplify introduction.
(Minibuffer File): Document tilde in minibuffer filenames.
(Minibuffer Edit): Mention that the prompt is read-only.  Describe how
to enter tabs, spaces, and question marks.  Describe behavior of C-a.
(Completion Example): Update example to current command list.
(Completion Options): Document `lazy' value of completion-auto-help.
Update contents of completion-ignored-extensions.
(Minibuffer History): Describe "future history" list.  State default
value of history-delete-duplicates.