LibCore: Add documentation to Stream functions + make parameter clearer

file_size was not very clear about what it was being used for, so I
switched it to say expected_file_size to make it clear that it's just a
heuristic.

Userland/Libraries/LibCore/Stream.cpp

@@ -52,7 +52,7 @@ ErrorOr<ByteBuffer> Stream::read_all(size_t block_size)
     return read_all_impl(block_size);
 }
 
-ErrorOr<ByteBuffer> Stream::read_all_impl(size_t block_size, size_t file_size)
+ErrorOr<ByteBuffer> Stream::read_all_impl(size_t block_size, size_t expected_file_size)
 {
     ByteBuffer data;
     data.ensure_capacity(file_size);