docs: client/llb: missing documentation for most llb exported identifiers

[profnandaa]

Problem

Quite a number of exported identifiers (types, variables, functions) are not documented within the source code.
This leaves the published package on pkg.go.dev/.../llb very scantily documented. For instance, there's no Overview, functions like Readonly, WriteTo, are not documented, etc:

It is best practice that any exported identifiers (especially publicly published) be documented.

Solution

We propose that we start slowly documenting, especially the major identifiers; file-by-file (to make it easy to resolve merge conflicts). For instance, we could start with adding a package Overview docstring in client/llb/definition.go and move to all exports in fileop.go, etc.

/assign @castrombithisamm

[castrombithisamm]

Thanks @profnandaa , I will take this up, but I might need some assistance.