doc: Fix warnings in `awful.screenshot` and `gears.shape`.

Those were written before the doc linting was merged.

lib/awful/screenshot.lua

@@ -440,15 +440,15 @@ end
 -- @tparam[opt=nil] screen|nil screen
 -- @propemits true false
 -- @see mouse.screen
--- @see awful.screen.focused
+-- @see screen.focused
 -- @see screen.primary
 
 --- Get screenshot client.
 --
 -- @property client
--- @tparam[opt=nil] client|nil client
+-- @tparam[opt=nil] client|nil client The client.
 -- @propemits true false
--- @see mouse.client
+-- @see mouse.current_client
 -- @see client.focus
 
 --- Get screenshot geometry.
@@ -891,6 +891,7 @@ end
 -- @method reject
 -- @tparam[opt=nil] string||nil reason The reason why it was rejected. This is
 --  passed to the `"snipping::cancelled"` signal.
+-- @noreturn
 -- @emits snipping::cancelled
 
 function module:reject(reason)

lib/gears/shape.lua

@@ -778,6 +778,7 @@ end
 -- @tparam number height The shape height
 -- @tparam[opt=5] number x_offset The shadow area horizontal offset.
 -- @tparam[opt=5] number y_offset The shadow area vertical offset.
+-- @noreturn
 function module.solid_rectangle_shadow(cr, w, h, x_offset, y_offset)
     x_offset, y_offset = x_offset or 5, y_offset or 5
     w, h = w - math.abs(x_offset), h - math.abs(y_offset)