clarify Base.isidentifier docs (false for operator symbols) (#42266)

Author: stevengj

base/show.jl

@@ -1315,7 +1315,8 @@ is_id_char(c::AbstractChar) = ccall(:jl_id_char, Cint, (UInt32,), c) != 0
      isidentifier(s) -> Bool
 
 Return whether the symbol or string `s` contains characters that are parsed as
-a valid identifier in Julia code.
+a valid ordinary identifier (not a binary/unary operator) in Julia code;
+see also [`Base.isoperator`](@ref).
 
 Internally Julia allows any sequence of characters in a `Symbol` (except `\\0`s),
 and macros automatically use variable names containing `#` in order to avoid