docs: add missing @return tags to every method

[tas50]

yard stats already reported 100% documented, but that only counts whether a
docstring exists. Measured over the .yardopts scope (which includes
--private --protected), 19 of 61 methods and attributes documented what they
took and what they did, but never what they handed back.

Before:

methods=61 missing_@param=0 missing_@return=19

After:

methods=61 missing_@param=0 missing_@return=0

@param coverage was already complete, and yard reports no warnings before
or after.

Private attr_readers

Most of the gap was private readers declared several to a line, such as
attr_reader :config, :client, :port, :logger. A single docstring above one of
those statements gives every reader the same @return, which would be wrong
for three of the four. YARD's attribute directives let each reader carry its
own type, but only when they follow the attr_reader statement -- attaching
them directly above it documents only the first name. They are therefore
grouped at the end of each class, with a comment saying why.

One correction

ServerOptions::SUFFIX_LENGTH was documented as "Random suffix appended to
generated names, plus its separator." It does not cover the separator:
generate_name budgets for separators separately, via parts.length. The
comment now says what the constant is.

Notes

• Comments only. No behaviour changes, and the diff contains no non-comment
  lines.
• bundle exec rake test: 69 examples, 0 failures, unchanged from main.
• bundle exec cookstyle --chefstyle (Cookstyle 9.0.0 / RuboCop 1.90.0):
  9 files inspected, no offenses detected.
• The doc and doc_coverage rake tasks and .yardopts already existed and
  are untouched; both tasks were run and still report 100.00% documented.
• Merges cleanly into feat: replace fog-cloudstack with cloudstack_client #62 with no conflicts (verified by a trial merge). Every
  docstring changed here describes a method feat: replace fog-cloudstack with cloudstack_client #62 keeps, and the wording stays
  accurate after that PR's rewrite.