docs: Document language items, as per #3342
remove stray mode on callback, seems to not change anything
core: Update libc docs to clarify usage
doc: Update tutorial description of core
auto merge of #5660 : brson/rust/doc, r=catamorphism
docs: Xfail test, it's just for illustration anyway
auto merge of #5662 : catamorphism/rust/docs, r=catamorphism

r? @brson
tidy version numbers and copyright dates
auto merge of #5667 : graydon/rust/remove-a-mode, r=catamorphism

It seems nobody can figure out whether this is _supposed to_ make a difference anymore, and in testing it seems to work either way, so I removed it. One less alarming warning during a fresh build.
auto merge of #5669 : graydon/rust/tweaks, r=thestinger

Just some minor cleanup of version strings and copyrights.