runtime: document GOMEMLIMIT in environment variables section

For #48409.

Change-Id: Ia6616a377bc4c871b7ffba6f5a59792a09b64809
Reviewed-on: https://go-review.googlesource.com/c/go/+/410734
Run-TryBot: Michael Pratt <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Chris Hines <[email protected]>
Reviewed-by: Russ Cox <[email protected]>

Author: prattmic

src/runtime/extern.go

@@ -18,8 +18,19 @@ The GOGC variable sets the initial garbage collection target percentage.
 A collection is triggered when the ratio of freshly allocated data to live data
 remaining after the previous collection reaches this percentage. The default
 is GOGC=100. Setting GOGC=off disables the garbage collector entirely.
-The runtime/debug package's SetGCPercent function allows changing this
-percentage at run time. See https://golang.org/pkg/runtime/debug/#SetGCPercent.
+[runtime/debug.SetGCPercent] allows changing this percentage at run time.
+
+The GOMEMLIMIT variable sets a soft memory limit for the runtime. This memory limit
+includes the Go heap and all other memory managed by the runtime, and excludes
+external memory sources such as mappings of the binary itself, memory managed in
+other languages, and memory held by the operating system on behalf of the Go
+program. GOMEMLIMIT is a numeric value in bytes with an optional unit suffix.
+The supported suffixes include B, KiB, MiB, GiB, and TiB. These suffixes
+represent quantities of bytes as defined by the IEC 80000-13 standard. That is,
+they are based on powers of two: KiB means 2^10 bytes, MiB means 2^20 bytes,
+and so on. The default setting is math.MaxInt64, which effectively disables the
+memory limit. [runtime/debug.SetMemoryLimit] allows changing this limit at run
+time.
 
 The GODEBUG variable controls debugging variables within the runtime.
 It is a comma-separated list of name=val pairs setting these named variables: