Add docstring compat annotations

Author: non-Jedi

base/io.jl

@@ -1016,6 +1016,9 @@ Return an iterable object yielding [`read(io, T)`](@ref).
 
 See also: [`skipuntil`](@ref), [`eachline`](@ref), [`readuntil`](@ref)
 
+!!! compat "Julia 1.6"
+    `eachof` requires Julia 1.6 or later.
+
 # Examples
 ```jldoctest
 julia> open("my_file.txt", "w") do io
@@ -1117,6 +1120,9 @@ characters from that character until the start of the next line are ignored.
 
 See also: [`readuntil`](@ref)
 
+!!! compat "Julia 1.6"
+    `skipchars` was renamed to `skipuntil` in Julia 1.6.
+
 # Examples
 ```jldoctest
 julia> buf = IOBuffer("    text")