doc: shorten character encoding introduction

Keep the introduction for Buffers and character encodings short and to the point. The current introduction doesn't provide much in the way of useful additional information, but it is a bit confusing in its wording. ("such as" seems like it ought to refer to "encoded characters" but it actually refers to character encodings, which are not mentioned in the sentence. It may be arguable as to whether "hex-encoded" is in fact a character encoding, whether it should be stylized as "Hex-encoded" or not, and whether it should be spelled out as "Hexadecimal-encoded". None of that information is particularly useful to the end user at this point in the text. Omitting it simplifies and improves the documentation.)

Additionally, the section is now wrapped to 80 characters.

Checklist

• documentation is changed or added
• commit message follows commit guidelines