docs for erf(x,y) (#224)

Author: cossio

docs/src/functions_overview.md

@@ -34,6 +34,7 @@ Here the *Special Functions* are listed according to the structure of [NIST Digi
 | Function | Description |
 |:-------- |:----------- |
 | [`erf(x)`](@ref SpecialFunctions.erf)         | [error function](https://en.wikipedia.org/wiki/Error_function) at ``x`` |
+| [`erf(x,y)`](@ref SpecialFunctions.erf)       | accurate version of ``\operatorname{erf}(y) - \operatorname{erf}(x)`` |
 | [`erfc(x)`](@ref SpecialFunctions.erfc)       | complementary error function, i.e. the accurate version of ``1-\operatorname{erf}(x)`` for large ``x`` |
 | [`erfcinv(x)`](@ref SpecialFunctions.erfcinv) | inverse function to [`erfc()`](@ref SpecialFunctions.erfc) |
 | [`erfcx(x)`](@ref SpecialFunctions.erfcx)     | scaled complementary error function, i.e. accurate ``e^{x^2} \operatorname{erfc}(x)`` for large ``x`` |

src/erf.jl

@@ -51,6 +51,10 @@ Compute the error function of ``x``, defined by
 \quad \text{for} \quad x \in \mathbb{C} \, .
 ```
 
+    erf(x, y)
+
+Accurate version of `erf(y) - erf(x)` (for real arguments only).
+
 External links: [DLMF](https://dlmf.nist.gov/7.2.1), [Wikipedia](https://en.wikipedia.org/wiki/Error_function).
 
 See also: [`erfc(x)`](@ref SpecialFunctions.erfc), [`erfcx(x)`](@ref SpecialFunctions.erfcx),
@@ -64,11 +68,6 @@ See also: [`erfc(x)`](@ref SpecialFunctions.erfc), [`erfcx(x)`](@ref SpecialFunc
 """
 erf
 
-@doc raw"""
-    erf(x, y)
-
-Compute the difference `erf(y) - erf(x)`, in a numerically sound way (only for real arguments).
-"""
 function erf(x::Real, y::Real)
     if 0 ≤ x && 0 ≤ y
         erfc(x) - erfc(y)