Renamed new 1.8.0-pre.3 method GetMagnitude to GetControlMagnitude to avoid confusion

[chris-massie]

Description

Processors such as AxisDeadzone or ScaleVector2 can cause the action's value (obtained with InputAction.ReadValue<TValue>) to be different from the control's magnitude (obtained with InputAction.GetMagnitude). This change makes it explicit that the magnitude is based on the active control to avoid confusion from users.

Changes made

Renamed method added with #1795 in this current 1.8.0-pre.3 that has not yet been released yet. Since it isn't public yet, we should be able to rename.

Checklist

Before review:

• Changelog entry added.
  • Explains the change in Changed, Fixed, Added sections.
  • For API change contains an example snippet and/or migration example.
  • FogBugz ticket attached, example ([case %number%](https://issuetracker.unity3d.com/issues/...)).
  • FogBugz is marked as "Resolved" with next release version correctly set.
• Tests added/changed, if applicable.
  • Functional tests Area_CanDoX, Area_CanDoX_EvenIfYIsTheCase, Area_WhenIDoX_AndYHappens_ThisIsTheResult.
  • Performance tests.
  • Integration tests.
• Docs for new/changed API's.
  • Xmldoc cross references are set correctly.
  • Added explanation how the API works.
  • Usage code examples added.
  • The manual is updated, if needed.

During merge:

• Commit message for squash-merge is prefixed with one of the list:
  • NEW: ___.
  • FIX: ___.
  • DOCS: ___.
  • CHANGE: ___.
  • RELEASE: 1.1.0-preview.3.

[ekcoh]

LGTM

[ekcoh]

@chris-massie I have now merged this PR.