Documentation: Shift and Ctrl keys for drawing boxes and box/ellipse rulers

[mejiasmario]

Summary

Since 0.30.6, pressing Shift while drawing a box centers it on the first point and Ctrl confines it to a square. The same keys apply to "Box" and "Ellipse" rulers. The manual doesn't mention this yet: "Doing Measurements" still says that Shift/Ctrl constrain the direction for every ruler, and "Creating A Box" doesn't mention the keys. I ran into this myself in issue #2440.

This PR:

• measure.xml: adds a paragraph on the "Box" and "Ellipse" rulers and their Shift/Ctrl behavior, and a pointer to it in the paragraph about direction constraints.
• create_box.xml: adds a paragraph on Ctrl (square) and Shift (centered).

The wording follows the code in antService.cc and edtBoxService.cc: both keys work independently and can be combined, and they apply only while drawing, not when moving a point of an existing ruler.

Test Plan

• Built on Linux (Ubuntu, Qt5) and checked both pages in the help browser.
• ut_runner layHelpIndexTest passes.