Add cross-references between readuntil and skipchars docstring

Part 1 of addressing #36132

Author: non-Jedi

base/io.jl

@@ -433,6 +433,8 @@ The delimiter can be a `UInt8`, `AbstractChar`, string, or vector.
 Keyword argument `keep` controls whether the delimiter is included in the result.
 The text is assumed to be encoded in UTF-8.
 
+See also: [`skipchars`](@ref)
+
 # Examples
 ```jldoctest
 julia> open("my_file.txt", "w") do io
@@ -1113,6 +1115,8 @@ Advance the stream `io` such that the next-read character will be the first rema
 which `predicate` returns `false`. If the keyword argument `linecomment` is specified, all
 characters from that character until the start of the next line are ignored.
 
+See also: [`readuntil`](@ref)
+
 # Examples
 ```jldoctest
 julia> buf = IOBuffer("    text")