Add comments about the purpose of the test, and about the static assert.

Author: mstorsjo

libcxx/test/std/input.output/file.streams/fstreams/ifstream.members/offset_range.pass.cpp

@@ -8,6 +8,9 @@
 
 // <fstream>
 
+// Test that we can seek using offsets larger than 32 bit, and that we can
+// retrieve file offsets larger than 32 bit.
+
 // XFAIL: target={{.*}}-windows{{.*}}
 
 #include <fstream>
@@ -60,6 +63,8 @@ void test_tellg(std::streamoff total_size) {
 }
 
 int main(int, char**) {
+  // This test assumes and requires that std::streamoff is larger than
+  // 32 bit - this is not required in the standard itself.
   static_assert(sizeof(std::streamoff) > 4);
   test_tellg(0x100000042ULL);
   return 0;