pkg/ottl: add section about design principles (open-telemetry#29424)

**Description:**

Drawing inspiration from
https://github.com/bazelbuild/starlark#design-principles and
https://github.com/google/cel-spec/blob/master/doc/langdef.md#overview,
add a brief section about design principles.

The aim of this is to ensure OTTL is and remains safe for execution of
untrusted programs in multi-tenant systems, where tenants can provide
their own OTTL programs.

---------

Co-authored-by: Evan Bradley <[email protected]>

pkg/ottl/LANGUAGE.md

@@ -2,6 +2,13 @@
 
 The OTTL grammar includes function invocations, Values and Boolean Expressions. These parts all fit into a Statement, which is the basis of execution in the OTTL.
 
+### Design principles
+
+OTTL is intended as a domain-specific language (DSL) for telemetry mutation and generation,
+and is not intended to be used as a general-purpose programming language.
+
+OTTL has been designed to work directly with [pdata](https://github.com/open-telemetry/opentelemetry-collector/tree/main/pdata), but can operate on other data formats.
+
 ### Editors
 
 Editors are functions that transform the underlying telemetry payload. They may return a value, but typically do not. There must be a single Editor Invocation in each OTTL statement.
@@ -288,4 +295,4 @@ It is possible to update the Value in a telemetry field using a Setter. For read
 
 ## Logging inside a OTTL function
 
-To emit logs inside a OTTL function, add a parameter of type [`component.TelemetrySettings`](https://pkg.go.dev/go.opentelemetry.io/collector/component#TelemetrySettings) to the function signature. The OTTL will then inject the TelemetrySettings that were passed to `NewParser` into the function. TelemetrySettings can be used to emit logs.
+To emit logs inside a OTTL function, add a parameter of type [`component.TelemetrySettings`](https://pkg.go.dev/go.opentelemetry.io/collector/component#TelemetrySettings) to the function signature. The OTTL will then inject the TelemetrySettings that were passed to `NewParser` into the function. TelemetrySettings can be used to emit logs.

pkg/ottl/ottlfuncs/README.md

@@ -8,6 +8,18 @@ This document contains documentation for both types of OTTL functions:
 - [Functions](#functions) that transform telemetry.
 - [Converters](#converters) that provide utilities for transforming telemetry.
 
+## Design principles
+
+For the standard OTTL functions described in this document, we specify design principles to ensure they are always
+secure and safe for use:
+
+- Built-in OTTL functions may not access the file system, network, or any other I/O devices.
+- Built-in OTTL functions may share information only through their parameters and results.
+- Built-in OTTL functions must be terminating; they must not loop forever.
+
+OTTL functions are implemented in Go, and so are only limited by what can be implemented in a Go program.
+User-defined OTTL functions may therefore not adhere the above principles.
+
 ## Working with functions
 
 Functions generally expect specific types to be returned by `Paths`.