cleanup and clarification of unified/managed memory docs

[briankoco]

Associated JIRA ticket number/Github issue number

N/A

What type of PR is this? (check all applicable)

• Refactor
• Feature
• Bug Fix
• Optimization
• Documentation Update
• Continuous Integration

What were the changes?

• Explain concept of managed memory in relation to unified memory
• Clarify that managed memory via system allocators is not officially supported on CDNA1
• Fix hipHostRegister() managed memory table entries

Why are these changes needed?

Some of the unified memory documentation is incorrect, and performance implications of managed memory were not sufficiently captured

Updated CHANGELOG?

• Yes
• No, Does not apply to this PR.

Added/Updated documentation?

• Yes
• No, Does not apply to this PR.

Additional Checks

• I have added tests relevant to the introduced functionality, and the unit tests are passing locally.
• Any dependent changes have been merged.

[adeljo-amd]

LGTM, I don't have access to approving PRs here. I've invited @randyh62 to also take a look

[adeljo-amd]

@briankoco Is there a reason why this was closed instead of it being merged in? Should we prepare a PR from our side with your changes?

[briankoco]

I believe the changes have been merged. Github was not allowing me to merge, with an error about the branch being unable to rebase due to conflicts. I followed the command line approach of checking out docs/develop and pulling in the changes from bjk-dev manually, and it seemed to work

[adeljo-amd]

Ah okay, I see the commits in docs/develop now