libcamera: Define the threading model

Document the design of libcamera's threading support, and prepare to
document thread-safety of classes and functions with a doxygen alias
command.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

Documentation/Doxyfile.in

@@ -239,7 +239,9 @@ TAB_SIZE               = 4
 # newlines (in the resulting output). You can put ^^ in the value part of an
 # alias to insert a newline as if a physical newline was in the original file.
 
-ALIASES                =
+ALIASES                = "context=\xrefitem context \"Thread Safety\" \"Thread Safety\""
+ALIASES               += "threadbound=\ref thread-bound \"thread-bound\""
+ALIASES               += "threadsafe=\ref thread-safe \"thread-safe\""
 
 # This tag can be used to specify a number of word-keyword mappings (TCL only).
 # A mapping has the form "name=value". For example adding "class=itcl::class"