Miscellaneous documentation additions.

Added notes explaining how [expr, ..expr] form is used, targeted at
individuals like me who thought it was more general and handled
dynamic repeat expressions.  (I left a TODO for this section in a
comment, but perhaps that is bad form for the manual...)

Added example of `do` syntax with a function of arity > 1; yes, one
should be able to derive this from the text above it, but it is still
a useful detail to compare and contrast against the arity == 1 case.

Added example of using for expression over a uint range, since someone
who is most used to write `for(int i; i < lim; i++) { ... }` will
likely want to know how to translate that form (regardless of whether
it happens to be good style or not for their use-case).

Added note about the semi-strange meaning of "fixed size" of vectors
in the vector type section.
Fixes mozilla/rust#3612.
Spelling fixes; replaced `size` with `capacity` in few places.
Cleanup wording around the awkward "and so on is . ." in the README.txt.
Updated to reflect alpha-rename of constant/static items section.
auto merge of #5553 : pnkfelix/rust/doc-fixes-for-0.6-incoming, r=sanxiyn