Add isunorded docstring and compat admonitions

Author: cmcaine

base/operators.jl

@@ -213,6 +213,15 @@ largest values and `isgreater` defines a descending total order with `NaN` and
 This is unexported. Types should not usually implement this function. Instead, implement `isless`.
 """
 isgreater(x, y) = isunordered(x) || isunordered(y) ? isless(x, y) : isless(y, x)
+
+"""
+    isunordered(x)
+
+Return true if `x` is a value that is not normally orderable, such as `NaN` or `missing`.
+
+!!! compat "Julia 1.7"
+    This method requires Julia 1.7 or later.
+"""
 isunordered(x) = false
 isunordered(x::AbstractFloat) = isnan(x)
 isunordered(x::Missing) = true

base/reduce.jl

@@ -775,6 +775,9 @@ are multiple maximal points, then the first one will be returned.
 
 Values are compared with `isless`.
 
+!!! compat "Julia 1.7"
+    This method requires Julia 1.7 or later.
+
 # Examples
 
 ```jldoctest
@@ -828,6 +831,9 @@ are multiple minimal points, then the first one will be returned.
 
 `NaN` is treated as less than all other values except `missing`.
 
+!!! compat "Julia 1.7"
+    This method requires Julia 1.7 or later.
+
 # Examples
 
 ```jldoctest
@@ -881,6 +887,9 @@ If there are multiple maximal values for `f(x)` then the first one will be found
 
 Values are compared with `isless`.
 
+!!! compat "Julia 1.7"
+    This method requires Julia 1.7 or later.
+
 # Examples
 ```jldoctest
 julia> argmax(abs, -10:5)
@@ -926,6 +935,9 @@ If there are multiple minimal values for `f(x)` then the first one will be found
 
 `NaN` is treated as less than all other values except `missing`.
 
+!!! compat "Julia 1.7"
+    This method requires Julia 1.7 or later.
+
 # Examples
 ```jldoctest
 julia> argmin(sign, -10:5)