[addrman] Add doxygen comment to AddrMan::Add()

Does not document the return value since we're going to fix the
semantics in a future commit.

src/addrman.h

@@ -69,7 +69,14 @@ class AddrMan
     //! Return the number of (unique) addresses in all tables.
     size_t size() const;
 
-    //! Add addresses to addrman's new table.
+    /**
+     * Attempt to add one or more addresses to addrman's new table.
+     *
+     * @param[in] vAddr           Address records to attempt to add.
+     * @param[in] source          The address of the node that sent us these addr records.
+     * @param[in] nTimePenalty    A "time penalty" to apply to the address record's nTime. If a peer
+     *                            sends us an address record with nTime=n, then we'll add it to our
+     *                            addrman with nTime=(n - nTimePenalty). */
     bool Add(const std::vector<CAddress>& vAddr, const CNetAddr& source, int64_t nTimePenalty = 0);
 
     //! Mark an entry as accessible, possibly moving it from "new" to "tried".