doc: add default value of special characters in mroonga_escape
doc: add default value of special characters in mroonga_escape

mroonga/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,naoa/mroonga,mroonga/mroonga,mroonga/mroonga,yoku0825/mroonga,yoku0825/mroonga