[lldb][Docs] Add equivalent of GDB's "skip" to command map

https://sourceware.org/gdb/current/onlinedocs/gdb.html/Skipping-Over-Functions-and-Files.html

We can't emulate all the features of that command but we can
skip a function by name with some extra steps.

As far as I know this only matches function name unlike GDB
that can filter on file and line and so on:
```
target.process.thread.step-avoid-regexp -- A regular expression defining functions step-in won't stop in.
```

It's likely it's got some corner cases that don't work, maybe
inlining, but it doesn't seem worth going into it here.

I don't think we can chain lldb interpreter commands, so I
have shown the steps separately.

Author: DavidSpickett

lldb/docs/use/map.rst

@@ -235,6 +235,23 @@ Do a source level single step in the currently selected thread
   (lldb) step
   (lldb) s
 
+Ignore a function when doing a source level single step in
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. code-block:: shell
+
+  (gdb) skip abc
+  Function abc will be skipped when stepping.
+
+.. code-block:: shell
+
+  (lldb) settings show target.process.thread.step-avoid-regexp
+  target.process.thread.step-avoid-regexp (regex) = ^std::
+  (lldb) settings set target.process.thread.step-avoid-regexp (^std::)|(^abc)
+
+Get the default value, make it into a capture group, then add another capture
+group for the new function name.
+
 Do a source level single step over in the currently selected thread
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~