doc: add memory safety section & unsafe {...} example (#5804)

doc/docs.md

@@ -70,6 +70,7 @@ you can do in V.
     * [vfmt](#vfmt)
     * [Profiling](#profiling)
 * [Advanced](#advanced)
+    * [Memory-unsafe code](#memory-unsafe-code)
     * [Calling C functions from V](#calling-c-functions-from-v)
     * [Debugging generated C code](#debugging-generated-c-code)
     * [Conditional compilation](#conditional-compilation)
@@ -1873,6 +1874,51 @@ fn main(){
 
 # Advanced Topics
 
+## Memory-unsafe code
+
+Sometimes for efficiency you may want to write low-level code that can potentially
+corrupt memory or be vulnerable to security exploits. V supports writing such code, 
+but not by default.
+
+V requires that any potentially memory-unsafe operations are marked intentionally.
+Marking them also indicates to anyone reading the code that there could be
+memory-safety violations if there was a mistake.
+
+Examples of potentially memory-unsafe operations are:
+
+* Pointer arithmetic
+* Pointer indexing
+* Conversion to pointer from an incompatible type
+* Calling certain C functions, e.g. `free`, `strlen` and `strncmp`.
+
+To mark potentially memory-unsafe operations, enclose them in an `unsafe` block:
+
+```v
+// allocate 2 uninitialized bytes & return a reference to them
+mut p := unsafe { &byte(malloc(2)) }
+p[0] = `h` // Error: pointer indexing is only allowed in `unsafe` blocks
+unsafe {
+    p[0] = `h`
+    p[1] = `i`
+}
+p++ // Error: pointer arithmetic is only allowed in `unsafe` blocks
+unsafe {
+    p++ // OK
+}
+assert *p == `i`
+```
+
+Best practice is to avoid putting memory-safe expressions inside an `unsafe` block,
+so that the reason for using `unsafe` is as clear as possible. Generally any code 
+you think is memory-safe should not be inside an `unsafe` block, so the compiler 
+can verify it.
+
+If you suspect your program does violate memory-safety, you have a head start on 
+finding the cause: look at the `unsafe` blocks (and how they interact with 
+surrounding code).
+
+* Note: This is work in progress.
+
 ## Calling C functions from V
 
 ```v