Expand the esp8266 hardware debugging limitations section (esp8266#8879)

It's trivial to run into a problem caused by the single hardware breakpoint
limitation, but the existing documentation doesn't explicitly address this
situation (and search results for the error message aren't particularly
helpful).

This commit updates the documentation with an explicit example of what
happens when a breakpoint expression matches multiple locations, along with
a possible workaround.

doc/gdb.rst

@@ -380,4 +380,33 @@ breakpoint) command in GDB while debugging instead of the more common
 ``break`` command, since ``thb`` will remove the breakpoint once it is
 reached automatically and save you some trouble.
 
+Because of the single hardware breakpoint limitation, you must pay careful
+attention to the output from ``gdb`` when you set a breakpoint. If your
+breakpoint expression matches multiple locations, as in this example:
 
+.. code:: bash
+
+    (gdb) break loop
+    Breakpoint 1 at 0x40202c84: loop. (2 locations)
+
+Then you will be unable to ``continue``:
+
+.. code:: bash
+
+    (gdb) cont
+    Continuing.
+    Note: automatically using hardware breakpoints for read-only addresses.
+    Warning:
+    Cannot insert hardware breakpoint 1.
+    Could not insert hardware breakpoints:
+    You may have requested too many hardware breakpoints/watchpoints.
+
+You can resolve this situation by deleting the previous breakpoint and
+using a more specific breakpoint expression:
+
+.. code:: bash
+
+    (gdb) delete
+    Delete all breakpoints? (y or n) y
+    (gdb) break mysketch.ino:loop
+    Breakpoint 2 at 0x40202c84: file .../mysketch.ino, line 113.