How To Document an Object Oriented Design in Text

For my next project, I’m looking to document my Object Oriented design in simple text before jumping the gun to code it up. I want to do this for two reasons.

ISTQB terminology question (Defect)

According to ISTQB (and few more sources + wiki ), a defect/bug is the actual cause of error in software, e.g. incorrect statement, logical or semantic error. The actual definion is: a flaw in the system or component that could lead to the failure.
But what about specification bugs? I cannot relate to it. Specification bugs are quite common but if the programmer implements software according to spec with a bug, it is not his fault (IMHO). But then the definion could not apply and I am sure it must have been addressed somehow. Could you help me to understand this?

How can a non-technical person learn to write a spec for small projects?

How can a non-technical person learn to write specs for small projects?

Estimating time for planning and technical design using Evidence Based Scheduling

I’m at the beginning of a development project in a large organization.

bug: deviation from requirements vs deviation from expectations

I am not clear on this one. No matter the terminology, in the end the software fault/bug causes (according to a lot of sources):

What is the process of planning software called? Or what is the job title of someone who does software planning?

What is the process of planning software called?

Best practices for making header file? [closed]

Closed 10 years ago.

General solution to solve different sports results in different languages

I currently have some code that checks if squash and tennis scores are valid, both in javascript and PHP.

Custom HTML Tags: Are there any specifications stating a standard way to handle them?

It seems like for years they’ve just been given default styling and inline display. Is there a spec somewhere that has dictated this? I’ve looked over the RFC’s but I’m not particularly good with RFC-ese, and I didn’t notice anything anywhere.

Is a functional spec a part of the System requirement spec?

I wonder, sources like wikipedia or templates I found shows that Functional spec is a part of System requirement documents. I always thought that SRD is just overall decsription of the system, with all functional and non functional requirements. Yet I thought that Functional spec is more detailed and it is a separate document, while SRD is high level customer-created description (how is this one called then?)
Could anyone help to make this clear for me?