Fix instructions

As per the tests, it is allowed to not being able to form a word from the fragments, in which case it should return 0. <!-- This is an auto-generated comment: release notes by coderabbit.ai --> ## Summary by CodeRabbit * **Documentation** * Updated documentation to clarify that it may not always be possible to construct the word from the given fragments, and to reflect the correct behavior when this occurs. <!-- end of auto-generated comment: release notes by coderabbit.ai -->