Added note to BLAS.[g|s]et_num_threads about Apple Accelerate not supporting it (#1195)

Per the documentation for `AppleAccelerate.jl`, `BLAS.get_num_threads()`
returns a hard coded default when using `AppleAccelerate`. This PR
simply adds a warning message when this function is called to alert the
user in the console itself.

Author: ajinkya-k

src/blas.jl

@@ -129,6 +129,10 @@ Set the number of threads the BLAS library should use equal to `n::Integer`.
 
 Also accepts `nothing`, in which case julia tries to guess the default number of threads.
 Passing `nothing` is discouraged and mainly exists for historical reasons.
+
+!!! note
+    Some BLAS libraries, such as Apple Accelerate, cannot be configured to use a fixed number of threads.
+    For these backends, `set_num_threads()` is a no-op. See also [`get_num_threads`](@ref).
 """
 set_num_threads(nt::Integer)::Nothing = lbt_set_num_threads(Int32(nt))
 function set_num_threads(::Nothing)
@@ -148,6 +152,10 @@ Get the number of threads the BLAS library is using.
 
 !!! compat "Julia 1.6"
     `get_num_threads` requires at least Julia 1.6.
+
+!!! note
+    Some BLAS libraries, such as Apple Accelerate, cannot be configured to use a fixed number of threads.
+    For these backends, `get_num_threads()` always returns `1`. See also [`set_num_threads`](@ref).
 """
 get_num_threads()::Int = lbt_get_num_threads()