Lower our max memory when generating documentation to 1GB.

It turns out that some 32-bit JVMs on Windows can't handle 1536MB (1.5GB), even though in practice, we're nowhere near the limit. The main reason for setting a maximum is to avoid system-dependent defaults which sometimes are way too low.
Merge pull request #1466 from AnalyticalGraphicsInc/adjustMaxMemory

Lower our max memory when generating documentation to 1GB.