move 'session-use-file-storage' server option to more appropriate section

[kevinushey]

It currently exists in the 'www' section:

rstudio/src/cpp/server/server-options.json

Lines 201 to 207 in 7b760a9

	{
	"name": "session-use-file-storage",
	"memberName": "sessionUseFileStorage_",
	"type": "bool",
	"defaultValue": true,
	"description": "Whether to use the file system to store metadata about the session storage or the internal database. Setting this to false may require special network configuration. See [Session Storage](../server_management/session_storage.html) for more information."
	},