unicode shortcuts documentation

this is in response to https://github.com/JuliaLang/julia/issues/7914

did i miss anything?

i think it would be nice to reiterate in the Functions section of the docs that unicode can be used for function names, but did not see a great place to do so.  any ideas?
Perhaps useful to say these can also be tab-completed?
```
julia> \al[TAB]
\aleph    \allequal  \alpha
```