[fix](doc) dev: correct wrong expected outputs in five string-function examples

[boluor]

Backport of #3860 to dev (docs/ + i18n/zh-CN/.../current/).

The same five string-function example pages carry an expected result that does not match the server; each is wrong in both EN and ZH. Verified on a Doris master build (doris-0.0.0-trunk-97b77e6cda).

Page	Documented	Correct (cluster)	Why
ends-with.md	ENDS_WITH('testing','test') = 1	0	"testing" does not end with "test"
reverse.md	irànnaumuḍḍ	irànnamuḍḍ	spurious extra u; actual reverse of ḍḍumannàri
mask-last-n.md	Hello你好nnn (header Hello你好123)	Helloṭṛnnn	result block was for a different input than the example SQL mask_last_n('Helloṭṛ123', 9)
position.md	13	12	'hello' starts at character position 12
mask.md	Xxxx999	Axxx999	example demonstrates a custom upper-case replacement 'ABC', so T → A, not default X

Cluster verification (Doris master build)

ENDS_WITH('test','test'), ENDS_WITH('testing','test')   -> 1 , 0
REVERSE('ṭṛì ḍḍumai'), REVERSE('ḍḍumannàri')            -> iamuḍḍ ìṛṭ , irànnamuḍḍ
mask_last_n('Helloṭṛ123', 9)                            -> Helloṭṛnnn
POSITION('ḍḍumai' IN 'ṭṛì ḍḍumai hello'),
    POSITION('hello','ṭṛì ḍḍumai hello',8)              -> 5 , 12
mask('Test123','ABC','xyz','999')                       -> Axxx999

🤖 Generated with Claude Code