-variable to show its value in a tooltip (@pxref{Tooltips}); this takes
-effect in the GUD interaction buffer, and in all source buffers with
-major modes listed in the variable @code{gud-tooltip-modes}. If the
-variable @code{gud-tooltip-echo-area} is non-@code{nil}, values are
-shown in the echo area instead of a tooltip.
-
- When using GUD Tooltip mode with @kbd{M-x gud-gdb}, you should note
-that displaying an expression's value in GDB can sometimes expand a
-macro, potentially causing side effects in the debugged program. If
-you use the @kbd{M-x gdb} interface, this problem does not occur, as
-there is special code to avoid side-effects; furthermore, you can
-display macro definitions associated with an identifier when the
-program is not executing.
+variable, a function, or a macro (collectively called
+@dfn{identifiers}) to show their values in tooltips
+(@pxref{Tooltips}). Alternatively, mark an identifier or an
+expression by dragging the mouse over it, then leave the mouse in the
+marked area to have the value of the expression displayed in a
+tooltip. The GUD Tooltip mode takes effect in the GUD interaction
+buffer, and in all source buffers with major modes listed in the
+variable @code{gud-tooltip-modes}. If the variable
+@code{gud-tooltip-echo-area} is non-@code{nil}, or if you turned off
+the tooltip mode, values are shown in the echo area instead of a
+tooltip.
+
+ When using GUD Tooltip mode with @kbd{M-x gud-gdb}, displaying an
+expression's value in GDB can sometimes expand a macro, potentially
+causing side effects in the debugged program. For that reason, using
+tooltips in @code{gud-gdb} is disabled. If you use the @kbd{M-x gdb}
+interface, this problem does not occur, as there is special code to
+avoid side-effects; furthermore, you can display macro definitions
+associated with an identifier when the program is not executing.