[mlir] Update docs referencing OpTrait::Symbol.

Since https://reviews.llvm.org/D78522, Symbol is not a Trait itself.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D88512

mlir/docs/Interfaces.md

@@ -228,3 +228,7 @@ format of the header for each interface section goes as follows:
          - RegionKind::Graph - represents a graph region without control flow semantics
          - RegionKind::SSACFG - represents an [SSA-style control flow](LangRef.md#modeling-control-flow) region with basic blocks and reachability
      - `hasSSADominance(unsigned index)` - Return true if the region with the given index inside this operation requires dominance.
+
+##### SymbolInterfaces
+
+*   `SymbolOpInterface` - Used to represent [`Symbol`](SymbolsAndSymbolTables.md#symbol) operations which reside immediately within a region that defines a [`SymbolTable`](SymbolsAndSymbolTables.md#symbol-table).

mlir/docs/SymbolsAndSymbolTables.md

@@ -37,10 +37,10 @@ link, or use, to the symbol. An example of a `Symbol` operation is
 
 ### Defining a Symbol
 
-A `Symbol` operation may use the `OpTrait::Symbol` trait to provide the
-necessary verification and accessors, but this is not required as some
-operations, such as `module`, conditionally define a symbol. `Symbol`s must have
-the following properties:
+A `Symbol` operation should use the `SymbolOpInterface` interface to provide the
+necessary verification and accessors; it also supports
+operations, such as `module`, that conditionally define a symbol. `Symbol`s must
+have the following properties:
 
 *   A `StringAttr` attribute named
     'SymbolTable::getSymbolAttrName()'(`sym_name`).

mlir/docs/Traits.md

@@ -267,13 +267,6 @@ associated with that memory reference.
 This trait provides APIs and verifiers for operations with regions that have a
 single block that must terminate with `TerminatorOpType`.
 
-### Symbol
-
-*   `OpTrait::Symbol` -- `Symbol`
-
-This trait is used for operations that define a
-[`Symbol`](SymbolsAndSymbolTables.md#symbol).
-
 ### SymbolTable
 
 *   `OpTrait::SymbolTable` -- `SymbolTable`