bpo-33642 and bpo-33679: Revise and condense blurbs (GH-7500)

Misc/NEWS.d/next/IDLE/2018-05-24-20-42-44.bpo-33642.J0VQbS.rst

-IDLE: Only display actual code context lines.
-Instead of displaying a fixed number of lines, some blank, Code Context
-now displays the variable number of actual context lines.  When there
-are no context lines, it shows a single blank line to indicate that the
-feature is turned on.
-
-The Code Context configuration option is changed from 'numlines'
-(default 3) to 'maxlines' (default 15) to avoid possible interference
-between user settings for the old and new versions of Code Context.
+Display up to maxlines non-blank lines for Code Context.
+If there is no current context, show a single blank line.

Misc/NEWS.d/next/IDLE/2018-05-29-07-14-37.bpo-33679.MgX_Ui.rst

-IDLE: Re-enable color configuration for Code Context.
-The difference from before is that the settings are now on the
-Highlights tab instead of the Extensions tab and only change one theme
-at a time instead of all themes.  The default for light themes is black
-on light gray, as before.  The default for the IDLE Dark theme is white
-on dark gray, which better fits the dark theme.
-
-When one starts IDLE from a console and loads a custom theme without
-definitions for 'context', one will see a warning message on the console.
-To stop the warning, go to Options => Configure IDLE => Highlights,
-select the custom theme if not selected already, select 'Code Context',
-and select foreground and background colors.
+Enable theme-specific color configuration for Code Context.
+Use the Highlights tab to see the setting for built-in themes
+or add settings to custom themes.