hush: document bugs in [[ =~ ]] comparison

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

shell/hush_test/hush-bugs/strops5_bug.right

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+4:no:1
+5:YES:0

shell/hush_test/hush-bugs/strops5_bug.tests

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+v='*.z'
+
+# Buggy:
+# the problem is that expansion rules of LHS and RHS of ~=
+# should not be the same: in RHS, "$v" and "*" should escape metas
+# (currently "$v" does not),
+# but in LHS, they should _not_ do that
+# (currently "*" does). Thus these cases fail:
+[[ a.z == "$v" ]]; echo 4:no:$?	# BUG: "$v" expands to *.z
+[[ "*".z == ?.z ]]; echo 5:YES:$?	# BUG: "*" expands to \*

shell/hush_test/hush-test2/strops5.right

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+1:YES:0
+2:no:1
+3:YES:0

shell/hush_test/hush-test2/strops5.tests

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+v='*.z'
+[[ a.z = *.z ]]; echo 1:YES:$?
+[[ a.z = "*".z ]]; echo 2:no:$?
+[[ a.z == $v ]]; echo 3:YES:$?
+
+# Buggy:
+# the problem is that expansion rules of LHS and RHS of ~=
+# should not be the same: in RHS, "$v" and "*" should escape metas
+# (currently "$v" does not),
+# but in LHS, they should _not_ do that
+# (currently "*" does). Thus these cases fail:
+#[[ a.z == "$v" ]]; echo 4:no:$?	# BUG: "$v" expands to *.z
+#[[ "*".z == ?.z ]]; echo 5:YES:$?	# BUG: "*" expands to \*