Lift and document indexing heap size

[jryans]

When getting an index writer, there is a magic number for the amount of heap that Tantivy should use:

seshat/src/index.rs

Lines 217 to 227 in 71e17fa

	pub fn get_writer(&self) -> Result<Writer, tv::Error> {
	Ok(Writer {
	inner: self.index.writer(50_000_000)?,
	body_field: self.body_field,
	topic_field: self.topic_field,
	name_field: self.name_field,
	event_id_field: self.event_id_field,
	room_id_field: self.room_id_field,
	})
	}
	}

It be nice to make this value more prominent at the top of the file (or similar) with a comment explaining how it was determined.