Add missing QLDoc

Author: owen-mc

go/ql/lib/semmle/go/dataflow/SSA.qll

@@ -198,6 +198,11 @@ class SsaExplicitDefinition extends SsaDefinition, TExplicitDef {
 
   override Location getLocation() { result = this.getInstruction().getLocation() }
 
+  /**
+   * Gets the first instruction that the value of this `SsaDefinition` can
+   * reach without passing through any other instructions, but possibly through
+   * phi nodes.
+   */
   IR::Instruction getAFirstUse() { firstUse(this, result) }
 }
 
@@ -413,4 +418,11 @@ DataFlow::Node getASimilarReadNode(DataFlow::Node node) {
   )
 }
 
+/**
+ * Gets an instruction such that  `pred` and `result` form an adjacent
+ * use-use-pair of the same`SsaSourceVariable`, that is, the value read in
+ * `pred` can reach `result` without passing through any other use or any SSA
+ * definition of the variable except for phi nodes and uncertain implicit
+ * updates.
+ */
 IR::Instruction getAnAdjacentUse(IR::Instruction pred) { adjacentUseUse(pred, result) }