Document Suggestion Quality column in speller tables

Add explanation of the S (Suggestion Quality) badge that shows:
- First%: percentage where correct word is first suggestion
- Top5%: percentage where correct word is in top 5
- Tests: number of typo test cases evaluated

Include color-coded quality thresholds (green/yellow/red/black)
with direct link to source code where thresholds are defined.

Author: snomos

proof/spelling/SpellerOverview.md

@@ -24,6 +24,18 @@ Being in the **Production** group means the speller has been tested and is consi
 
 This objective classification ensures transparency and gives language teams clear upgrade criteria.
 
+**Suggestion Quality (S):** The tables below include a "Suggestion Quality" column showing how well each spellchecker provides correct spelling suggestions. The badge displays three values: **First% | Top5% | Tests**
+
+- **First%**: Percentage of typos where the correct word is the first suggestion
+- **Top5%**: Percentage of typos where the correct word is in the top 5 suggestions  
+- **Tests**: Number of typo test cases evaluated (formatted as "k" for thousands)
+
+Badge colors indicate overall quality based on [these thresholds](https://github.com/giellalt/giella-core/blob/main/scripts/make-spellerbadge-json.sh#L84-L97):
+- 🟢 **Green** (good): First ≥ 80% AND Top5 ≥ 90% AND Tests ≥ 1000
+- 🟡 **Yellow** (medium): First ≥ 60% AND Top5 ≥ 70% AND Tests ≥ 500
+- 🔴 **Red** (poor): First ≥ 40% AND Top5 ≥ 50% AND Tests ≥ 100
+- ⚫ **Black** (very poor): Below red thresholds
+
 ## [![Maturity: Production](https://img.shields.io/badge/Maturity-Production-brightgreen.svg)](../../MaturityClassification.html) Production
 
 <div id="prod_spellers" ></div>