Clarify GetProperty API doc (facebook#12829)

Summary:
**Context/Summary:** as titled since https://github.com/facebook/rocksdb/blob/9eebaf11cbd875435b572f05f0378ecdb761cc74/db/internal_stats.cc#L1162.

Pull Request resolved: facebook#12829

Test Plan: no code change

Reviewed By: pdillinger

Differential Revision: D59243565

Pulled By: hx235

fbshipit-source-id: 074137b29bb12d9d965d154626a3289f85a39c52

include/rocksdb/db.h

@@ -1334,9 +1334,10 @@ class DB {
 
   // DB implementations export properties about their state via this method.
   // If "property" is a valid "string" property understood by this DB
-  // implementation (see Properties struct above for valid options), fills
-  // "*value" with its current value and returns true.  Otherwise, returns
-  // false.
+  // implementation (see Properties struct above for valid options) and the DB
+  // is able to get and fill "*value" with its current value, then return true.
+  // In all the other cases (e.g, "property" is an invalid "string" property, IO
+  // errors ..), it returns false.
   virtual bool GetProperty(ColumnFamilyHandle* column_family,
                            const Slice& property, std::string* value) = 0;
   virtual bool GetProperty(const Slice& property, std::string* value) {