Small fixes to function aliases and descriptions

[fionaliao]

• sin - this was added to function metadata but was missing from Description()
• exp - description name was set to "add" instead of "exp"
• Added pct as an function alias for asPercent as graphite-web does https://github.com/graphite-project/graphite-web/blob/dca59dc72ae28ffdae659232c10c60aa598536eb/webapp/graphite/render/functions.py#L6011
• xFilesFactor - this was added to function metadata but was missing from Description()
• Corrected the Function and Name fields in the function descriptions for scaleAfterTimestamp and sinFunction