fix one more issue in commit 9b5b197695ebe05bb95cf60a0b028e3677529c8f
base/operators.jl: remove harmful (incorrect) fallback for cmp.

This was causing cmp(Uint128,Uint128) to give wrong answers. The
other more general fallback that already exists is better.
Make doctests pass

Reflects lots of small changes in printouts and error messages, line numbers, etc.

Nontrivial changes:
- Bump JuliaDoc submodule
- The control flow documentation had to be reformatted slightly to make
  doctests pass.  The output of Task(...) now prints out a memory
  address which is clearly not reproducible, so the output is
  suppressed.
- Some roundoff printing issues in complex-and-rational-numbers?
- isequal(NaN,NaN32) == true
- It also looks like sqrt(-1) no longer prints a helpful error message
  about trying sqrt(complex(-1))...?
- strings.rst, line 239: "ERROR: BoundsError() in getindex at
  ascii.jl:11" no longer prints out traceback??
- control-flow.rst, line 638: UndefVarError->MyUndefVarError to prevent
  overwriting Base UndefVarError

WARNNG: Because union types appear to be unsorted on output, sometimes
Union(a,b) is printed out as Union(b,a). This causes the doctests to
fail nondeterministically, e.g.

Failed example:
    super(Union(Float64,Int64))
Expected:
    ERROR: `super` has no method matching super(::Type{Union(Float64,Int64)})
Got:
    ERROR: `super` has no method matching super(::Type{Union(Int64,Float64)})
Update sqrt(-1) doctests
Fix Union printout orders
Update line numbers for square
Demote doctests containing Unions to ordinary code snippets

This is a workaround for the nondeterministic order of Union printouts - they can be output as Union(a,b) or Union(b,a), which confuses the Sphinx doctest parser.
Update Travis CI build to try building docs