Clarify bufferSize parameter description in XML

[ViveliDuCh]

Correct bufferSize parameter descriptions in StreamReader and StreamWriter XML docs

• Clarify StreamReader.bufferSize sets the internal byte buffer; StreamWriter.bufferSize sets the internal char buffer
• Remove incorrect buffer size units and misleading references
• Omit secondary buffer details for clarity
• Align docs with implementation and recent runtime changes

Note that both classes allocate both buffer types, but bufferSize only directly affects one.
Documentation update in response to issue dotnet/runtime#118748 and code changes in PR dotnet/runtime#118958

[jozkee]

StreamWriter must be changed too, it describes bufferSize in terms of bytes but is actually in terms of chars.