Updates MemoryStream.reserve DDOC

Removes ambiguity as to whether `reserve` is relative to the entire stream or the cursor position.
Fix Issue 4859 - Another File.byChunk()
Merge pull request #1203 from andralex/4859

Fix Issue 4859 - Another File.byChunk()
Merge pull request #1669 from RommelVR/patch-1

Updates MemoryStream.reserve DDOC