Only update symbols on current thread when breaking or stepping in GDB.

mojo/devtools/common/README.md

 # Devtools
 
 Unopinionated tools for **running**, **debugging** and **testing** Mojo apps.
 
 ## Install
 
 ```
 git clone https://github.com/domokit/devtools.git
 ```
 
@@ skipping 21 matching lines @@
 
 ```sh
 mojo_debug tracing start
 mojo_debug tracing stop [result.json]
 ```
 
 The trace file can be then loaded using the trace viewer in Chrome available at
 `about://tracing`.
 
 #### GDB
-It is possible to inspect a Mojo Shell process using GDB. The `debugger` script
+It is possible to inspect a Mojo Shell process using GDB. The `mojo_debug` script
 can be used to launch GDB and attach it to a running shell process (android
 only):
 
 ```sh
 mojo_debug gdb attach
 ```
 
+Once started, GDB will first stop the Mojo Shell execution, then load symbols
+from loaded Mojo applications. Please note that this initial step can take some
+time (up to several minutes in the worst case).
+
+After each execution pause, GDB will update the set of loaded symbols based on
+the selected thread only. If you need symbols for all threads, use the
+`update-symbols` GDB command:
+```sh
+(gdb) update-symbols
+```
+
+If you only want to update symbols for the current selected thread (for example,
+after changing threads), use the `current` option:
+```sh
+(gdb) update-symbols current
+```
+
 #### Android crash stacks
 When Mojo shell crashes on Android ("Unfortunately, Mojo shell has stopped.")
-due to a crash in native code, `debugger` can be used to find and symbolize the
+due to a crash in native code, `mojo_debug` can be used to find and symbolize the
 stack trace present in the device log:
 
 ```sh
 mojo_debug device stack
 ```
 
 ### devtoolslib
 
 **devtoolslib** is a Python module containing the core scripting functionality
 for running Mojo apps: shell abstraction with implementations for Android and
 Linux and support for apptest frameworks. The executable scripts in devtools are
 based on this module. One can also choose to embed the functionality provided by
 **devtoolslib** in their own wrapper.
 
 ## Development
 
 The library is canonically developed [in the mojo
 repository](https://github.com/domokit/mojo/tree/master/mojo/devtools/common),
 https://github.com/domokit/devtools is a mirror allowing to consume it
 separately.