fix(docs): Add set_provider! to the documentation

Author: kunzaatko

docs/src/fft.md

@@ -20,4 +20,5 @@ FFTW.plan_dct
 FFTW.plan_idct
 FFTW.plan_dct!
 FFTW.plan_idct!
+FFTW.set_provider!
 ```

docs/src/index.md

@@ -15,12 +15,10 @@ Pkg.add("FFTW")
 
 from the Julia REPL.
 
-Users with a build of Julia based on Intel's Math Kernel Library (MKL) can use MKL
-for FFTs by setting a preference in their top-level project by either using the
-`FFTW.set_provider!()` method, or by directly setting the preference using
-[`Preferences.jl`](https://github.com/JuliaPackaging/Preferences.jl).  Note that this
-choice will be recorded for the current project, and other projects that wish to use
-MKL for FFTs should also set that same preference.
-Note further that MKL provides only a subset of the functionality provided by FFTW. See
-Intel's [documentation](https://software.intel.com/en-us/mkl-developer-reference-c-using-fftw3-wrappers)
-for more information about potential differences or gaps in functionality.
+Users with a build of Julia based on Intel's Math Kernel Library (MKL) can use MKL for FFTs by setting a preference in
+their top-level project by either using the [`FFTW.set_provider!()`](@ref) method, or by directly setting the preference
+using [`Preferences.jl`](https://github.com/JuliaPackaging/Preferences.jl).  Note that this choice will be recorded for
+the current project, and other projects that wish to use MKL for FFTs should also set that same preference. Note further
+that MKL provides only a subset of the functionality provided by FFTW. See Intel's
+[documentation](https://software.intel.com/en-us/mkl-developer-reference-c-using-fftw3-wrappers) for more information
+about potential differences or gaps in functionality.