docs: add links to string context documentation

operators are an everyday thing in the Nix language, and this page will
hopefully be consulted by many users.
string contexts are quite exotic, and not linking to the detailed
explanation will require readers to figure out manually what this is
about, or worse, skim over and run into problems later.

doc/manual/source/language/operators.md

@@ -102,7 +102,7 @@ The `+` operator is overloaded to also work on strings and paths.
 >
 > *string* `+` *string*
 
-Concatenate two [strings][string] and merge their string contexts.
+Concatenate two [strings][string] and merge their [string contexts](./string-context.md).
 
 [String concatenation]: #string-concatenation
 
@@ -128,7 +128,7 @@ The result is a path.
 
 > **Note**
 >
-> The string must not have a string context that refers to a [store path].
+> The string must not have a [string context](./string-context.md) that refers to a [store path].
 
 [Path and string concatenation]: #path-and-string-concatenation