[DomTree] Add docstring for `dominates(const Instruction*, const BasicBlock*)`. NFC

llvm/include/llvm/IR/Dominators.h

public:
   ///  * Non-instruction Defs dominate everything.
   ///  * Def does not dominate a use in Def itself (outside of degenerate cases
   ///    like unreachable code or trivial phi cycles).
   ///  * Invoke/callbr Defs only dominate uses in their default destination.
   bool dominates(const Value *Def, const Use &U) const;
   /// Return true if value Def dominates all possible uses inside instruction
   /// User. Same comments as for the Use-based API apply.
   bool dominates(const Value *Def, const Instruction *User) const;
-  // Does not accept Value to avoid ambiguity with dominance checks between
-  // two basic blocks.
+
+  /// Returns true if Def would dominate a use in any instruction in BB.
+  /// If Def is an instruction in BB, then Def does not dominate BB.
+  ///
+  /// Does not accept Value to avoid ambiguity with dominance checks between
+  /// two basic blocks.
   bool dominates(const Instruction *Def, const BasicBlock *BB) const;
 
   /// Return true if an edge dominates a use.
   ///
   /// If BBE is not a unique edge between start and end of the edge, it can
   /// never dominate the use.
   bool dominates(const BasicBlockEdge &BBE, const Use &U) const;
   bool dominates(const BasicBlockEdge &BBE, const BasicBlock *BB) const;