update --pkgimages=existing option in CLI section of manual (#53343)

Author: KristofferC

doc/src/manual/command-line-interface.md

@@ -173,7 +173,7 @@ The following is a complete list of command-line switches available when launchi
 |`--handle-signals={yes*\|no}`          |Enable or disable Julia's default signal handlers|
 |`--sysimage-native-code={yes*\|no}`    |Use native code from system image if available|
 |`--compiled-modules={yes*\|no\|existing|strict}` |Enable or disable incremental precompilation of modules. The `existing` option allows use of existing compiled modules that were previously precompiled, but disallows creation of new precompile files. The `strict` option is similar, but will error if no precompile file is found. |
-|`--pkgimages={yes*\|no}`               |Enable or disable usage of native code caching in the form of pkgimages|
+|`--pkgimages={yes*\|no|existing}`               |Enable or disable usage of native code caching in the form of pkgimages. The `existing` option allows use of existing pkgimages but disallows creation of new ones|
 |`-e`, `--eval <expr>`                  |Evaluate `<expr>`|
 |`-E`, `--print <expr>`                 |Evaluate `<expr>` and display the result|
 |`-L`, `--load <file>`                  |Load `<file>` immediately on all processors|