Fix broken link and other help improvements (closes #2760)

data/core.yaml

@@ -526,13 +526,13 @@ en:
     gps: |
       # GPS
 
-      GPS data is the most trusted source of data for OpenStreetMap. This editor
+      Collected GPS traces are one valuable source of data for OpenStreetMap. This editor
       supports local traces - `.gpx` files on your local computer. You can collect
       this kind of GPS trace with a number of smartphone applications as well as
       personal GPS hardware.
 
       For information on how to perform a GPS survey, read
-      [Surveying with a GPS](http://learnosm.org/en/beginner/using-gps/).
+      [Mapping with a smartphone, GPS, or paper](http://learnosm.org/en/mobile-mapping/).
 
       To use a GPX track for mapping, drag and drop the GPX file onto the map
       editor. If it's recognized, it will be added to the map as a bright purple
@@ -595,9 +595,8 @@ en:
       has a type assigned, the inspector will display fields with details about
       the feature like its name and address.
 
-      Below the fields you see, you can click icons to add other details,
-      like [Wikipedia](http://www.wikipedia.org/) information, wheelchair
-      access, and more.
+      Below the fields you see, you can click the 'Add field' dropdown to add
+      other details, like a Wikipedia link, wheelchair access, and more.
 
       At the bottom of the inspector, click 'Additional tags' to add arbitrary
       other tags to the element. [Taginfo](http://taginfo.openstreetmap.org/) is a