Bug: too wide unicode character ranges echo «Invalid range» error

[ZyX]

I need to test whether character is in range [\u00A0-\uD7FF] (actual collection 
has more items, see printable characters definition in YAML 1.2 RFC), but this 
throws «Invalid range» error. After some experiments, I found that character 
ranges that are wider then 0x100 also result in error:

    :echo ''=~'[\uD6FE-\uD7FF]'
    E16: Invalid range
    0
    :echo ''=~'[\uD6FF-\uD7FF]'
    0
    :echo ''=~'[\uD6FE-\uD7FE]'
    0

Tested on latest vim 7.3. This should be either fixed or at least mentioned in 
documentation.

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