Improve lint rule help

[IEncinas10]

Summary

Rule descriptions can be obtained by running verible-verilog-lint --help_rules=all, which is
not properly formatted for long descriptions. It also doesn't support showing examples, which might be useful for some complex rules. Current workaround is reading the source code and tests for a rule, which is not all that great.

Proposal

1. Fixing formatting for very long descriptions shouldn't be hard. Easiest solution is to break description strings with newlines and indentation after a certain threshold is past.
2. We might want to add an extra-verbose mode where rule help prints a few examples for a given rule.

---

If anyone wants to take this / ask for pointers, go ahead. A few tips for potential contributors unfamiliar with the tool / bazel build system.

1. bazel build //verilog/tools/lint:verible-verilog-lint compiles the CLI lint tool, which will be available at ./bazel-bin/verilog/tools/lint/verible-verilog-lint
2. verible-verilog-lint --help_rules=all / verible-verilog-lint --help_rules=<RULE_NAME>should print rules' help message.

Relevant snippets of code:

• verible/verilog/analysis/descriptions.h

  Line 38 in 14eed6a

  struct LintRuleDescriptor {

• verible/verilog/analysis/checkers/line_length_rule.cc

  Line 63 in 14eed6a

  const LintRuleDescriptor& LineLengthRule::GetDescriptor() {

  (every rule has a description, this is just an example)

• verible/verilog/analysis/verilog_linter.cc

  Line 362 in 14eed6a

  void GetLintRuleDescriptionsHelpFlag(std::ostream *os,