Add Info Text for Shared Memory Configuration in Session Launcher UI

[agatha197]

In the session launcher UI of Backend.AI, it is important for users to understand how the shared memory configuration affects the available main memory for their sessions. Currently, there seems to be a lack of clear information regarding this. To improve user experience and avoid confusion, the following enhancements are proposed:

1. Info Text Addition: Introduce an info text near the shared memory configuration setting. This text should explain that the amount of shared memory specified will be deducted from the total main memory available for the session. This clarification will help users make informed decisions about their shared/main memory allocation.
2. Warning Helper Text: If the remaining main memory (after allocating shared memory) is below a certain threshold, display a warning helper text. This warning should inform users that the available main memory might be insufficient for optimal session performance. It could also suggest reviewing the shared memory allocation if necessary.