Add accuracy note for 2-arg log (#18360)

* Add accuracy note for 2-arg log

Addresses #18298.

* update based on comments

Author: simonbyrne

base/math.jl

@@ -161,6 +161,18 @@ julia> log(4,8)
 julia> log(4,2)
 0.5
 ```
+
+!!! note
+    If `b` is a power of 2 or 10, `log2` or `log10` should be used, as these will
+    typically be faster and more accurate. For example,
+
+    ```jldoctest
+    julia> log(100,1000000)
+    2.9999999999999996
+
+    julia> log10(1000000)/2
+    3.0
+    ```
 """
 log(b::Number, x::Number) = log(promote(b,x)...)
 

doc/stdlib/math.rst

@@ -835,6 +835,18 @@ Mathematical Functions
        julia> log(4,2)
        0.5
 
+   .. note::
+      If ``b`` is a power of 2 or 10, ``log2`` or ``log10`` should be used, as these will typically be faster and more accurate. For example,
+
+      .. doctest::
+
+          julia> log(100,1000000)
+          2.9999999999999996
+
+          julia> log10(1000000)/2
+          3.0
+
+
 .. function:: log2(x)
 
    .. Docstring generated from Julia source