docs: Add documentation of -vast-snapshot-at CLI option.

Author: lkorenc

docs/GettingStarted/debug.md

@@ -32,4 +32,7 @@ vast-opt --pass-pipeline="pipeline-string"
 
 ### Debug Pipeline
 
-With the `-vast-debug` option, you get more detailed crash reports. It shows MLIR operations when there's an error and provides current stack traces.
+With the `-vast-debug` option, you get more detailed crash reports. It shows MLIR operations when there's an error and provides current stack traces.
+
+Sometimes it is needed to examine the results of conversion steps more closely to discover what went wrong. `-vast-snapshot-at=pass1;...;passN` will instrument conversion pipeline to store a result of `passN` into a file after it is applied. Name of the file will be in the form of: `basename.pass_name`.
+Passing `"*"` in the string will result in output after every step.

docs/Tools/vast-front.md

@@ -44,6 +44,10 @@ Additional customization options include:
 - `-vast-disable-vast-verifier`
   - Skips verification of the produced VAST MLIR module.
 
+- `vast-snapshot-at="pass1;...;passN`
+  - After each pass that was specified as an option store MLIR into a file (format is `src.pass_name`).
+  - `"*"` stores snapshot after every conversion.
+
 ## Pipelines
 
-WIP pipelines documentation
+WIP pipelines documentation