two @defgroup comment fixes

Issue #1103938: Menu constants doc not formatted right and @name misues changes improper @name directives to @defgroup in D6, and essentially back-ports D7 comments to do so.

A couple small changes to these comments were suggested -- see jhodgdon's a) and b) in comment #6 in that issue -- which should therefore be changed in D7/D8 as well.

In menu.inc:

/**
 * @defgroup menu_item_types Menu item types
 * @{
 * Definitions for various menu types.

"various menu types" should be "various menu item types"

And in pager.inc:

/**
 * @defgroup pagerpieces Pager pieces
 * @{
 * Use these pieces to construct your own custom pagers in your theme.

the first line should not be a command, but a description.

Patch coming.