Update documentation of avail_idx & used_idx

[andreeaflorescu]

Summary of the PR

The avail_idx and used_idx methods panic if the ordering is not the right one. As checking the ordering incurs a performance degradation, the VMM will be responsible for calling these functions only with the right ordering. We're now making this panic explicit by adding it to the public documentation.

Requirements

Before submitting your PR, please make sure you addressed the following
requirements:

• All commits in this PR are signed (with git commit -s), and the commit
  message has max 60 characters for the summary and max 75 characters for each
  description line.
• All added/changed functionality has a corresponding unit/integration
  test.
• Any newly added unsafe code is properly documented.

[jiangliu]

How abouting using "debug_assert()" instead of runtime checking?

[andreeaflorescu]

How abouting using "debug_assert()" instead of runtime checking?

@jiangliu this will still cause a panic. In case we don't want to fix this by checking the operation, than we can just leave it as is and add a comment to the function specifying that the function panics in case it's called with unsupported ordering. WDYT?

[jiangliu]

How abouting using "debug_assert()" instead of runtime checking?

@jiangliu this will still cause a panic. In case we don't want to fix this by checking the operation, than we can just leave it as is and add a comment to the function specifying that the function panics in case it's called with unsupported ordering. WDYT?

I feel it's a type of program error instead of a runtime error, so not sure about the runtime performance impact.