[Docs] mount/shallow: update: fix semantics description

Fixes #1405

Author: Lukas Kullmann

docs/api/ReactWrapper/update.md

@@ -1,10 +1,12 @@
 # `.update() => Self`
 
-Forces a re-render. Useful to run before checking the render output if something external
+Syncs the enzyme component tree snapshot with the react component tree. Useful to run before checking the render output if something external
 may be updating the state of the component somewhere.
 
 NOTE: can only be called on a wrapper instance that is also the root instance.
 
+NOTE: this does not force a re-render. Use `wrapper.setProps({})` to force a re-render.
+
 
 #### Returns
 

docs/api/ShallowWrapper/update.md

@@ -1,10 +1,12 @@
 # `.update() => Self`
 
-Forces a re-render. Useful to run before checking the render output if something external
+Syncs the enzyme component tree snapshot with the react component tree. Useful to run before checking the render output if something external
 may be updating the state of the component somewhere.
 
 NOTE: can only be called on a wrapper instance that is also the root instance.
 
+NOTE: this does not force a re-render. Use `wrapper.setProps({})` to force a re-render.
+
 
 #### Returns
 

docs/api/mount.md

@@ -192,7 +192,7 @@ A method that un-mounts the component.
 A method that re-mounts the component.
 
 #### [`.update() => ReactWrapper`](ReactWrapper/update.md)
-Calls `.forceUpdate()` on the root component instance.
+Syncs the enzyme component tree snapshot with the react component tree.
 
 #### [`.debug() => String`](ReactWrapper/debug.md)
 Returns a string representation of the current render tree for debugging purposes.

docs/api/shallow.md

@@ -200,7 +200,7 @@ Returns a wrapper representing the `wrappingComponent`, if one was passed.
 Returns the instance of the root component.
 
 #### [`.update() => ShallowWrapper`](ShallowWrapper/update.md)
-Calls `.forceUpdate()` on the root component instance.
+Syncs the enzyme component tree snapshot with the react component tree.
 
 #### [`.debug() => String`](ShallowWrapper/debug.md)
 Returns a string representation of the current shallow render tree for debugging purposes.