docs,README: improve/elaborate main prj description [no ci]

By just describing as 'style checking tool', one could have thought
of FSharpLint as a formatting tool (sort of an alternative to fantomas)
but we cover way more than that. And to give a better idea of what
kind of issues can be detected, we put three different kinds of rules
here.

Author: knocte

README.md

@@ -1,6 +1,11 @@
 # FSharpLint [![GitHub Actions Build Status](https://github.com/fsprojects/FSharpLint/workflows/CI/badge.svg)](https://github.com/fsprojects/FSharpLint/actions?query=branch%3Amaster)
 
-FSharpLint is a style checking tool for F#. It points out locations where a set of rules on how F# is to be styled have been broken.
+FSharpLint is a static analysis tool for F#. Like many other software linters, it analyzes your source code (via typed and untyped syntax trees), and then it points out locations where a set of rules on how F# is to be styled have been broken. There are various kinds of rules:
+
+* Conventions: name your public or private API/ABI members to respect project or solution-wide conventions.
+* Smells: get warned on logic that might pose maintainability problems or potential downsides.
+* Formatting: indent and structure your code to follow a specific set of cosmetic standards.
+
 The tool is configurable via JSON and can be run from a console app, or as an MSBuild task. It also provides an interface to easily integrate the tool into other software.
 
 The project aims to let the user know of problems through [matching user defined hints](https://fsprojects.github.io/FSharpLint/how-tos/rules/FL0065.html) a la [HLint](http://community.haskell.org/~ndm/hlint/), and also by using custom rules written in F# similar to the rules in [Mascot](http://mascot.x9c.fr/manual.html) and [StyleCop](https://github.com/StyleCop/StyleCop).

docs/content/index.md

@@ -6,7 +6,13 @@ menu_order: 1
 
 # Overview
 
-FSharpLint is a style checking tool for F#. It points out locations where a set of rules on how F# is to be styled have been broken.
+FSharpLint is a static analysis tool for F#. Like many other software linters, it analyzes your source code (via typed and untyped syntax trees), and then it points out locations where a set of rules on how 
+F# is to be styled have been broken. There are various kinds of rules:
+
+* Conventions: name your public or private API/ABI members to respect project or solution-wide conventions.
+* Smells: get warned on logic that might pose maintainability problems or potential downsides.
+* Formatting: indent and structure your code to follow a specific set of cosmetic standards.
+
 The tool is configurable via JSON and can be run from a console app, or as an MSBuild task. It also provides an interface to easily integrate the tool into other software.
 
 The project aims to let the user know of problems through [matching user defined hints](./how-tos/rules/FL0065.html)