doc: clarify Buffer.indexOf() and lastIndexOf()

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

doc

Description of change

Clarifies the buffer documentation to explain how type coercion works for indexOf() and lastIndexOf().

Fixes #9801 (closed)

@vsemozhetbyt @silverwind